Most mechanics charge double time for working Sunday, even if they had not work at all the previous week (which doesn't happen, but if it did). Retail is open weekend, but mechanics like just about everyone else wants to work their 9-5 monday-friday job so they get weekends off.

In the US, I've only lived in Chicago, Portland (OR), San Francisco, Dallas, NYC, and Auburn, but I don't think I have ever seen an automobile garage open on a Sunday unless it was a WalMart, Sam's, or Costco.