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
. 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 at the time) 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.)