Do you greet clients "Happy Memorial Day?"
It is generally considered inappropriate to say "Happy Memorial Day". Memorial Day is a solemn day of remembrance for service members who died in action, not a celebratory holiday like the Fourth of July.
Instead of "Happy Memorial Day," opt for more respectful alternatives:
"Have a meaningful Memorial Day."
"Wishing you a reflective Memorial Day."
"Thank you for remembering our fallen heroes."
Note: Since this holiday honors those who died in service, it is also advised not to thank living veterans for their service on this specific day; that is best reserved for Veterans Day. So yeah, don't do it if you learned your client is a US veteran, okay?