The Irish are very religious and never want to presume to put themselves before God. Weddings should not take place on Christmas, which is Jesus' day, nor during Lent. The saying goes:

Monday for health,
Tuesday for wealth,
Wednesday the best day of all,
Thursday for losses,
Friday for crosses and
Saturday no day at all.

In Irish tradition, weddings shouldn't happen on Sunday because this is the Lord's day. The dancing and partying that happens after a wedding would not be appropriate on a Sunday. Since Irish weddings can often stretch on for 2-3 days, you can see why they aren't pleased with a wedding that starts on Friday or Saturday either!

Irish Wedding Traditions