To wish someone a 'Happy Colombus Day' you could use the expression Buona festa di Cristoforo Colombo. Although the word 'happy' literally translates as felice, Italians are more likely to use the phrase buona festa di or 'good feast of...' to mark a holiday, as in Buona festa di San Giuseppe or 'Happy Saint Joseph's day!'
The celebration of Columbus Day is a tradition that marks the arrival of Christopher Columbus to the Americas in 1492. Although Columbus' birthplace was Genoa in Italy, his journey to America is not something that is held as being particularly significant in Italian culture. This could be partly because Genoa was, at the time of Columbus, an independent city state, and not part of a united 'Italy' as we know it. In fact, the phrase Hai scoperto l'America! or 'You've discovered America!' is a sarcastic expression used to mock someone when they have made a banal discovery or been surprised by something that was already common knowledge.