Are Mormons allowed to get married on a Sunday?

Temples are closed on Sundays, so there are no ordinances going on there, sealings or otherwise. As for civil marriages, I don't believe any government offices are open on Sundays either. If you were to do a wedding outside the temple, with a different priest or pastor, I guess that would be fine. You can't use a ward building because, hey, it's Sunday and there are church services and meetings.