The Collaborative International Dictionary of English v.0.48:

Lammas \Lam"mas\, n. [AS. hl[=a]mmesse, hl[=a]fm[ae]sse, loaf
   mass, bread feast, or feast of first fruits; hl[=a]f loaf +
   m[ae]sse mass. See Loaf, and Mass religious service.]
   The first day of August; -- called also Lammas day, and
   Lammastide.
   [1913 Webster] Lammergeier
   Lammergeir

WordNet (r) 3.0 (2006):

Lammas
    n 1: commemorates Saint Peter's miraculous deliverance from
         prison; a quarter day in Scotland; a harvest festival in
         England [syn: Lammas, Lammas Day, August 1]