Happy Memorial Day
It does seem juxtapositional to wish someone a "Happy Memorial Day", given the reason for the holiday.
Nevertheless, Memorial Day has become an informal marker to begin Summer, and is a reason for people to appreciate the sacrifices of others. I'm all in favor of expressing gratitude, and I believe gratitude is good.
I enjoy the feeling that Summer brings: People start to relax. There are fewer expectations on each other. There are vacations. There is swimming. People go outside willingly. All of those restorative things are also good.
Now, the reason we begin those things around the end of May is probably a little bit political. And the "start of Summer" jump-starts the season by about a month. (the "end of summer" also short-changes the season by about a month... so I guess there's equilibrium?) But, the Holiday is there to remind folks that there was a price to pay, and it was paid. The positive things that have accrued to us deserve to be acknowledged.
So, maybe the "Happy" part is acknowledging that I can have these happy things because of the suffering and loss of others. Not really saying that the day is happy. Just that the Day is a periodic reminder that the sacrifice has not been in vain. We are Happy (and Thank You!)!