Saturdays, yes. Sundays, no.
What sucks is that if you order something on their more expensive 3 day select option, which means they only deliever on business days, they don't include saturday.
Most of their services don't include Sunday as a day, so if you get second day air, ordered on a Saturday you might not (read: probably won't.) get the package until Tuesday.
I believe the only service that will deliver on Sundays is same day and custom critical services, which you obviously pay an exorbant amount for.