Armed Forces Day

The third Saturday in May

"Happy Armed Forces Day" or " Thank you for your service" 

Are appropriate greetings

Memorial Day

The last Monday in May

It is not appropriate to say. "Happy Memorial Day".

Don't say it.

Memorial Day needs to be a solemn remembrance of the brave Americans who have laid down their lives 

for the freedom we cherish. 

Veterans Day

The 11th day of November

"Thank you for your service"

Is the proper greeting.