How did the Romans congratulate a couple on their wedding day? The concepts of wedding and marriage were not quite what they are now back then, but I assume that celebrations and congratulations were not unheard of.
I can think of many possible nice things to say and I can put them in Latin, so the focus is on attested, especially multiply attested if any, congratulations.
I am currently on my way to a wedding, but this question is not urgent as the couple appears not to be very fluent in Latin and might not appreciate its use quite enough. (Note added later: No Latin ended up being used in the wedding.)