From Facebook Developer Wiki

why only april fools? why not any date/timeframe? this is silly. - 193700405 07:19, 3 April 2008 (PDT)

I believe this was done as an April Fool's joke. Possibly in response to the fb:is-it-christmas that was posted around the Christmas holiday.

Ok, but my point is still valid: why make "cute" tags like this when a generic version could actually be of some use for developers? - 193700405 02:51, 7 April 2008 (PDT)

It's a reasonable request, but an engineer here thinks it's not necessary since date checks can be done programmatically in your application.