Does usps ship on saturday?

I recently bought merchandise on ebay it was shiped by ups ground on fri but didn't get any traction cause ups doesn't ship on weekends so i was pondering as to why he did it i asked him why he didn't ship by the post office cause I believe usps has a 6day workweek meaning it would of shiped on sat he said usps don't ship on weekends where he lives i thought he was lying so not sure whose telling the truth