To wish someone a happy National Day, you can say:

"Happy National Day!"

"I wish you a happy National Day!"

"Celebrate National Day with joy and pride!"

"Wishing you a wonderful National Day celebration!"

"May your National Day be filled with happiness and celebration!"

You can also use National Day as an adjective, pke this:

"I'm sending you National Day blessings!"

"I hope you have a National Day filled with love and laughter!"

"On this National Day, may you be surrounded by the people you love!"

"May your National Day be as special as you are!"