Following a community discussion, the names of articles having to do with dates shall be in cardinal format — as seen in 25 December. However, redirects shall be maintained for the ordinal format, as in the case of 25th December.
Since the general goal on this wiki is to preference British English, this rule derives from a perception by the community that placing the day in front of the month is the "most British" way of handling dates. Additionally, the forum discussion stipulated that the cardinal form was simply used more often than the ordinal form in modern British English.
The cardinal form was also cited as being easier to use from a technical, coding standpoint.
That said, the ruling only applies to the names of the articles, and not to links made to those articles.
There is no consensus which prohibits other formats being used within the body of articles through pipe switching, since it can easily be proved that the British use "1 January", "January 1" and "January 1st" in various circumstances. It would be advisable, however, to use the same format within the confines of a single article.