In some years the celebration lasts three days. For some people, Irish or not, the celebration lasts all week.
Missoula and Helena celebrated St. Patrick's Day in 2011 back on Saturday, March 12, with their respective parades. That might be so they could stage a parade at all, since many (and I do mean MANY) of the people in Missoula and Helena will be in Butte for the real celebration.
Besides the parade, there is a morning mass at 8 a.m. in St. Patrick's Catholic Church and lots of beer drinking. (and I do mean....etc)