Under Jewish law, Saturday is a holy day during which no work is to be performed. Shabbos as it is referred to in Hebrew, begins at sundown on Friday and ends at sundown on Saturday (a shabbos meal would be held Friday evening with all the preparation being performed before sundown). In the movie, Walter claims to have only picked up The Dude's call mid-voicemail because it was clearly stated as an emergency (answering the phone being considered "work") in which case a Jew may break the observance of Shabbos. The Dude explains that he has finally figured out The Big Lebowski's scheme (as his mind grows clearer postcoitally), to which Walter continues to demand explanation of the "emergency" which would allow him to break the day of rest. We cut to Walter picking up The Dude only after The Dude threatens to leave the bowling team.
The Jewish day of rest comes up again when Donny approaches Walter and The Dude to announce that the dates of the bowling tournament had been posted. Furious at the news that their team is scheduled to "roll" on the holy day, Walter tirades against the league and the person in charge of scheduling. As we learn from Donny, Burkhalter is charge of scheduling who Walter then refers to as "that kraut".