&%%(diary-float 11 4 -1) American Thanksgiving
The 11 specifies November (the eleventh month), the 4 specifies Thursday (the fourth day of the week, where Sunday is numbered zero), and the -1 specifies "last" (1 would mean "first," 2 would mean "second," -2 would mean "second-to-last," and so on). The month can be a single month or a list of months. Thus you could change the 11 above to '(1 2 3) and have the entry apply to the last Thursday of January, February, and March. If the month is t, the entry applies to all months of the year.
Most generally, sexp diary entries can perform arbitrary computations to determine when they apply. See Sexp Diary Entries.
