A hallmark is a mark stamped on silver to verify that it is of sufficient quality (not too high a percentage of another metal in the alloy) -- the original hallmark was stamped at Goldsmiths' Hall in London, England. Other stamps were added to show what silversmith had made the item as well.

Hallmark is a card company founded in 1910 by a guy (still a teenager) named Joyce C. Hall. Originally he just sold postcards, but he and his brothers expanded into the greeting card business. Presumably the name is just a pun on his own last name.

(And they did not make up Mother's Day, whose history is detailed in that node, nor Valentine's Day, nor any major holiday -- Sweetest Day, maybe.)