The Christian season whose fasting rules were historically relaxed on St Patrick’s Day was Lent.
Saint Patrick’s Day falls on 17 March, during the season of Lent in many years. Historically, Christians observing Lenten discipline were expected to fast or abstain from certain foods and pleasures, but the feast day allowed a temporary relaxation of those rules. That exception helped make the holiday both religious and celebratory.
The relaxation traditionally included restrictions on eating and drinking alcohol. In Ireland, this contributed to the association between the feast and stout, whiskey, and other forms of public conviviality. It did not mean that all Christians everywhere followed identical rules: fasting practices varied by denomination, period, and local custom.
A common mix-up is to treat St Patrick’s Day as separate from the church calendar because modern parades and green clothing dominate public celebrations. Its date, however, is the traditional feast day of Ireland’s patron saint and remains closely connected with the Lenten calendar.