Do you work on Sundays?
What do you usually do at the weekend?
(Essential Grammar in Use)
Can the plural and singular forms be exchanged like these?
Do you work on the Sunday?
What do you usually do at weekends?
Yes, you can say on Sunday and on weekends, or at weekends. Informally, or in North American, you could also say "Do you work Sunday(s)?"
On Monday I do yoga.
As said from snailboat, at the weekend is British English, equivalent to on the weekend used in North American English.
The office is closed at the weekend.
The office is closed on the weekend.