Well, I live in a country (Israel) where Memorial Day pretty much takes over everyone's life (no entertainment and the media is devoted entirely to fallen soldiers' and their families' stories). No one would ever dream of wishing someone a "Happy Memorial Day". So I guess we just have a different way of looking at this particular day.
If your original comment had stated that you didn't understand the phrase and explained why, I seriously doubt anyone would have taken offense.
I'm a combat vet. I also have dead to mourn on Memorial Day. And yes, there's an inherent contradiction in saying "Happy" associated with "Memorial Day". We also have a lot of parades on Memorial Day - to honor those who have died. That doesn't mean that we don't enjoy going to the parades, or have this strange mix of grief at loss and gratitude for time spent with family and friends going on at the same time.