WordNet (r) 3.0 (2006):

call it a day
    v 1: stop doing what one is doing; "At midnight, the student
         decided to call it quits and closed his books" [syn: call
         it quits, call it a day]

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "call it a day":
   break, breathe, button up, call a strike, call it quits, cap,
   carry to completion, clean up, climax, close down, close out,
   close shop, close up, complete, conclude, crown, culminate, end,
   finalize, finish, finish off, finish up, get done, get it over,
   get through, get through with, go on strike, knock off, lay off,
   mop up, pause, perfect, recess, round out, secure, shut down,
   shut up shop, stop for breath, strike, take a break, take a recess,
   take a rest, take five, take ten, take time out, terminate,
   top off, top out, walk out, wind up, wrap up