Good question MMario, in the North East of England "Serious Drinking"
tends to take place on Friday & Saturday nights.
For this reason celebrations of more minor events (which St Patrick's day is In NE England) tend to take place on the Friday or Saturday closest to the actual event.
However for reasons I am at a loss to explain such St Patrick's day celebrations as there were locally took place on the Sunday(the 19th) this year.
There are precedents for this,these days St Patrick's Day in Dublin lasts the best part of a week.
So the answer to your question is the calendar is becoming very fluid.