It isn't a made-up occasion to sell cards. It's origins are in Christianity. It is the middle Sunday in Lent and originally the mother was the mother church in Jerusalem. In the middle ages, it began to be seen as applying to the Virgin Mary, mothers and then individual mothers and servants and work people would visit home and as the day is in spring, probably, collect any wild flowers they saw on the way as an offering to their mother. Now of course people buy cards, but the feast preceeded the cards by as much as 1,000 years. the made up occasion to sell cards is the Mother's day in May.
Why on earth did you order flowers for your MiL? If your H cares about his mother, he should make the effort and order them himself - and face the recriminations, if there are any, when he doesn't.