The Romans may or may not have wished each other a happy birthday. (If you know and have a reference, please post it on the How to Say Happy Birthday in Latin Submission page.) That doesn't mean we can't use the living Latin language to wish someone a happy birthday. The following seems to me to be the best way to express Happy Birthday in Latin:
felix sit natalis dies
Thanks to Laura Higley for this suggestion:
"I would use the accusative case, specifically, the accusative of exclamation
felicem diem natalem = Happy Birthday!
"Habeas felicitatem in die natus es!" is another email suggestion.
From the University of Missouri, a Greek major would say, literally, "Natalis laetus mihi!" or "Natalis laetus tibi!" depending on whether the greeting is to yourself or another.
From reader Kevin Hsu:
"Maybe it would be worth taking a look at other romance languages to get an insight -- in Catalan we say, 'Per molts anys' which would be literally translated as 'for many years'. In Romanian, they say 'la multi ani' which is something very similar..."
Steven LaPeruta writes:
"After reading your page dedicated to saying 'Happy Birthday' in Latin, especially the last comment suggesting that we should look at modern romance languages. Considering that Italian is the evolution of Latin we could use that as a starting point, 'Te faustus omnibus prosequor', using the Italian 'Ti faccio gli auguri' (auguri meaning wishes as opposed to birthday)."