Wikipedia:Citing sources
For example, if an edition of a book was first released in 2005 with an identical reprinting in 2007, date it to 2005. If substantive changes were made in a reprint, sometimes marked on the verso with "Reprinted with corrections", note the edition and append the corrected reprint year to it (e.g. "1st ed. reprinted with corrections 2005").
Editors should be aware that older sources (especially those in the public domain) are sometimes republished with modern publication dates; treat these as new publications. When this occurs and the citation style being used requires it, cite both the new and original publication dates, e.g.:

Darwin, Charles (1964) [1859]. On the Origin of Species (facsimile of 1st ed.). Harvard University Press.
This is done automatically in the {{citation}} and {{cite book}} templates when you use the |orig-date= parameter.
Alternately, information about the reprint can be appended as a textual note:

Boole, George (1854). An Investigation of the Laws of Thought on Which Are Founded the Mathematical Theories of Logic and Probabilities. Macmillan. Reprinted with corrections, Dover Publications, New York, NY, 1958.

Seasonal publication dates and differing calendar systems: Publication dates, for both older and recent sources, should be written with the goal of helping the reader find the publication and, once found, confirm that the correct publication has been located. For example, if the publication date bears a date in the Julian calendar, it should not be converted to the Gregorian calendar.
If the publication date was given as a season or holiday, such as "Winter" or "Christmas" of a particular year or two-year span, it should not be converted to a month or date, such as July–August or December 25. If a publication provided both seasonal and specific dates, prefer the specific one.

Additional annotation: In most cases it is sufficient for a citation footnote simply to identify the source (as described in the sections above); readers can then consult the source to see how it supports the information in the article. Sometimes, however, it is useful to include additional annotation in the footnote, for example to indicate precisely which information the source is supporting (particularly when a single footnote lists more than one source – see § Bundling citations and § Text–source integrity, below).
A footnote may also contain a relevant quotation from the source. This is especially helpful when the cited text is long or dense. A quotation allows readers to immediately identify the applicable portion of the reference. Quotes are also useful if the source is not easily accessible. However, caution should be exercised, as always, to avoid copyright violations.
In the case of non-English sources, it may be helpful to quote from the original text and then give an English translation. If the article itself contains a translation of a quote from such a source (without the original), then the original should be included in the footnote. (See the WP:Verifiability § Non-English sources policy for more information.)

Notes and references section: This section describes how to add footnotes and also describes how to create a list of full bibliography citations to support shortened footnotes.
The first editor to add footnotes to an article must create a dedicated citations section where they are to appear. Any reasonable name may be chosen. The most frequent choice is "References". Other options in diminishing order of popularity are, "Notes", "Footnotes", or "Works cited", although these are more often used to distinguish between multiple end-matter sections or subsections.
For an example of headings of a notes section, see the article Tezcatlipoca.

General references: A general reference is a citation to a reliable source that supports content, but is not linked to any particular text in the article through an inline citation. General references are usually listed at the end of the article in a "References" section, and are usually sorted by the last name of the author or the editor. General reference sections are most likely to be found in underdeveloped articles, especially when all article content is supported by a single source. The disadvantage of general references is that text–source integrity is lost, unless the article is very short. They are frequently reworked by later editors into inline citations.
The appearance of a general references section is the same as those given above in the sections on short citations and parenthetical references. If both cited and uncited references exist, their distinction can be highlighted with separate section names, e.g., "References" and "General references".

How to create the list of citations: With some exceptions discussed below, citations appear in a single section containing only the <references /> tag or the {{Reflist}} template. For example:

References: {{Reflist}}

or

References: <references />

The footnotes will then automatically be listed under that section heading. Each numbered footnote marker in the text is a clickable link to the corresponding footnote, and each footnote contains a caret that links back to the corresponding point in the text.

Scrolling lists, or lists of citations appearing within a scroll box, should never be used. This is because of issues with readability, browser compatibility, accessibility, printing, and site mirroring.
If an article contains a list of general references, this is usually placed in a separate section, titled, for example, "References". This usually comes immediately after the section(s) listing footnotes, if any. (If the general references section is called "References", then the citations section is usually called "Notes".)

Separating citations from explanatory footnotes: If an article contains both footnoted citations and other (explanatory) footnotes, then it is possible (but not necessary) to divide them into two separate lists using footnotes groups. The explanatory footnotes and the citations are then placed in separate sections, called (for example) "Notes" and "References", respectively.
Another method of separating explanatory footnotes from footnoted references is using {{efn}} for the explanatory footnotes. The advantage of this system is that the content of an explanatory footnote can in this case be referenced with a footnoted citation. When explanatory footnotes and footnoted references are not in separate lists, {{refn}} can be used for explanatory footnotes containing footnoted citations.

Duplicate citations: Combine precisely duplicated full citations, in keeping with the existing citation style (if any). In this context "precisely duplicated" means having the same content, not necessarily identical strings ("The New York Times" is the same as "NY Times"; different access-dates are not significant). Do not discourage editors, particularly inexperienced ones, from adding duplicate citations when the use of the source is appropriate, because a duplicate is better than no citation. But any editor should feel free to combine them, and doing so is the best practice on Wikipedia.
Citations to different pages or parts of the same source can also be combined (preserving the distinct parts of the citations), as described in Help:References and page numbers. Any method that is consistent with the existing citation style (if any) may be used, or consensus can be sought to change the existing style. Some tools are linked below.

Citation style: While citations should aim to provide the information listed above, Wikipedia does not have a single house style, though citations within any given article should follow a consistent style. A number of citation styles exist, including those described in the Wikipedia articles for Citation, APA style, ASA style, MLA style, The Chicago Manual of Style, Author-date referencing, the Vancouver system and Bluebook.
Although nearly any consistent style may be used, avoid all-numeric date formats other than YYYY-MM-DD, because of the ambiguity concerning which number is the month and which the day. For example, 2002-06-11 may be used, but not 11/06/2002. The YYYY-MM-DD format should in any case be limited to Gregorian calendar dates where the year is after 1582. Because it could easily be confused with a range of years, the format YYYY-MM (for example: 2002-06) is not used.
For more information on the capitalization of cited works, see Wikipedia:Manual of Style/Capital letters § All caps and small caps.

Variation in citation methods: Editors should not attempt to change an article's established citation style, merely on the grounds of personal preference or to make it match other articles, without first seeking consensus for the change.
As with spelling differences, it is normal practice to defer to the style used by the first major contributor or adopted by the consensus of editors already working on the page, unless a change in consensus has been achieved. If the article you are editing is already using a particular citation style, you should follow it; if you believe it is inappropriate for the needs of the article, seek consensus for a change on the talk page. If you are the first contributor to add citations to an article, you may choose whichever style you think best for the article. However, since 5 September 2020, inline parenthetical referencing is a deprecated citation style on English-language Wikipedia.
If all or most of the citations in an article consist of bare URLs, or otherwise fail to provide needed bibliographic data –  such as the name of the source, the title of the article or web page consulted, the author (if known), the publication date (if known), and the page numbers (where relevant) –  then that would not count as a "consistent citation style" and can be changed freely to insert such data.