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.
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.