Everyone and their grandmother has live sales for black friday you're thinning possible customers. As for why St Patty's? You would be the only one to answer that assuming you're marketing or it's a coincidence more are buying them at this moment than another.
You could be just growing and catching a break, don't look a gift horse too close in the mouth.
My guess is because this is around the time people start getting tax refunds in the US if they filed immediately after it was available to do so. Black Friday is a shopping holiday whose name carries it alone for a lot of people, so it's easier to feel like you're getting a 'deal' if you do not know any better.
Sales are often based on availability. No point in putting things on sale if you can sell all you can get at regular price. Last fall there were shortages everywhere. As supply grows you will see more things on sale.