Feast of St. Matthias, May 14

From Catholic Online

After the Ascension, the apostles needed a replacement for Judas Iscariot. Peter set the criteria - a man who had been a disciple of Christ from His baptism by John through the Ascension. Two guys met the criteria - Matthias and Joseph (also called (Barsabbas). Since the apostles did not profess to know God's will in the matter, they prayed over it. No...well, maybe. But they DID cast lots. And Matthias was the winner!

Therefore, according to Wikipedia:

According to old tradition, St. Matthias's Day is said to be the luckiest day of the year. This is because Matthias was the saint who was chosen by lot to replace Judas Iscariot It has therefore been seen as a good day on which to buy lottery tickets or to participate in activities such as that.

Because you don't violate the first commandment if it's done in honor of a saint.

St. Matthias is the patron saint of the diocese of Gary, Indiana and Great Falls, Montana.  Makes you wonder how they picked him.