An earthquake would ruin my St. Patricks' Day plans. I would still be upset by the events of the earthquake. It would be difficult for me to rebound from that terror and move on with my day with any semblance of normalcy. I would; however, try to carry out my plans because it would not benefit anyone if I sat home in fear. It would ruin my plans in that I would not be in a very festive mood.
If there was an earthquake large enough to do significant damage to my city the last thing on my mind would be knocking back a few green beers. I'd need to make sure that my family and friends were fine and even if everyone I cared about was fine, it still wouldn't be really appropriate to party in a damaged city.
An earthquake on St. Patrick's Day wouldn't ruin my plans because the tremors aren't planned events. If anything, earthquake would make the day memorable. Most quakes are harmless and rattle walls and make chandeliers swing a few inches. The only way a tremor ruins my plans is if it is very strong in my local area.
It seems to me that any day's plans would be ruined if there were an earthquake of major proportions because one's whole world would be shaken up and there could be a lot of damage. As for St. Patrick's day, that is pretty much just another day for many of us.