Not impossible to find Sunday cancels but much less common than the other days of the week. A browse through the Postal Law and Regulation volumes indicates that Sunday window hours seem to have disappeared in 1912, but the mail still moved on Sundays.
Likely sources:
Special Delivery mail.
RPO mail & Large cities handled mail all the time behind the scenes.
Many large cities have/had airport mail facilities with a public window that operated 24/7/365.
Stamp shows and other pictorial cancels at festivals, etc.
Part of the challenge is that mail sent during our lifetimes is generally too common and inexpensive to be a separate lot on
ebay, nor do describers consistently list the year, let alone the postmark date, in lot descriptions. It took quite a while for me to find a cover from the day I was born (a Saturday).
Be sure to search for permutations of the date. For example if I were looking for Christmas day of 1950, I would search:
("December 25 1950", "Dec 25 1950 ", 25 December 1950", "25 Dec 1950", "12/25/1950", "12/25/50"). And for special delivery (and similar) covers I might even search for the day or two before hoping to find a transit or receiving mark.