Is it appropriate to call a professor for a recommendation on the weekend?
No -- it's totally inappropriate.









Seriously. Think about it. Unless you're at a school where profs phone grad students at their home on weekends to inquire about a late paper, it's totally inappropriate. 


Note: Many grant agencies / admissions committees know that we (the faculty) are thorough flakes and couldn't be trusted to tie our own shoelaces in the morning. They usually give a few days of extra time for faculty references to come in after the deadline.  Others, however, are very strict on deadlines and thus faculty need to be properly shepherded into getting their materials in time.