Green’s Dictionary of Slang

Sunday (punch) n.

[on the pattern of Sunday, therefore best, suit]
[1910s+] (orig. US)

a very hard or knockout blow; thus as v., to hit with such a blow; thus Sunday-puncher, one who hits with such a blow.

In phrases

cop a Sunday (on) (v.)

[1920s+] (US) to hit someone very hard.