A surprise addition to the holiday theme puzzle series! There are six "days" represented in the puzzle with their names put into boxes, thus leading to the revealer, BOXINGDAY. Excellent. The actual choices of days are interesting: we get one actual holiday with Patriots Day, which should appeal to our New England readers. Then there's Election Day, about which the less said the better.
The other four examples are a motley crew indeed. One event that happens once every four years (Leap Day), one event that happens once every week (Hump Day), one event that happens weekly in the right season (game day), and one event that may never happen at all in portions of the country, or possibly in the whole country some day if climate change continues (snow day). I enjoyed the variety, although I could see how some might nitpick.
The boxes make for triple-checked letters, and I would say that Mr. Markey has done a pretty remarkable job of keeping things as smooth as possible around those areas. Yes, there's ANATOLE and NIEBUHR, both of which are not Monday level answers, and SILOING is not really a word, is it? But otherwise, I like ARMLESS and EARFLAP, and any mention of XENA is a win in my book.
My error came at the cross of ESTA and SEABEES. I tried an E there, which even in retrospect seems to make sense. After all, the name comes from Construction Battalion (thus C.B.) and could be transliterated with a double-E. Anyway.
1A: Go out, as the tide (EBB) gets a D+ for crosswordese and blandness.
On the whole, a good Monday.