Ta’anit Bechorot (Fast of the Firstborn) falls on Friday, April 6, 2012. An earlier version of the calendar we publish incorrectly had this minor fast on Thursday.
When this fast day falls on Friday, we do in fact observe the fast on Friday, even though Shabbat follows immediately.
It is only in years when the date of the fast falls on Shabbat that the fast is moved. In this case, Ta’anit Bechorot is moved back to Thursday, because it is preferable not to fast immediately before Shabbat.
Source: Mishnah Berurah: the classic commentary to Shulchan Aruch Orach Chayim, comprising the laws of daily Jewish conduct, Volume V (B), Pesach. §470: “That the first-born fast on Erev Pesach.”