I prefer mornings at Union Square because I stalk the chefs a bit and land good produce. On the other hand. . . the end of the day is also good because of markdowns, which can lead to unexpected riches.
At busy places that are sort of touristy, like Zabar's, morning also allows you to be free of looky-loos and just do efficient shopping.
Eataly, which is highly underrated as a premium grocery store, is open until ELEVEN PM which is incredible. They start packing up the fish and meat around 10:30. This is a fave late-night joint for me and Rob when we are bored and want to take a lil stroll and maybe check off an errand while we're at it.
Kalustyans should be ashamed of itself for not opening until 10 am. How can such a valuable resource for chefs open so late in the day? Mad annoying.
That being said, Dual Spices, on First Avenue, doesn't open until 11 am. But it cuts the difference by not closing until 1 am for some reason. Perhaps they are hoping for drunk post-bar grocery shopping? It's great to shop here after an event in the East Village or visiting B29 Teahouse. However, there's a good amount of cross contamination in the nuts here so be careful if you have allergies (they also always taste a little like the smell of spices – not necessarily a bad thing haha!). High turnover.
I've never been able to get my ass up early enough to see when Chinatown as a whole starts doing business, but my guess is 8 am-ish is when you'll start seeing the Forsyth Street underpass fill with fruit and vegetable vendors. Chinatown vegetable vendors close up around 6-8ish, depending on the weather and the sunshine. There are some late night full-scale grocery store holdouts on Mott Street in what I refer to as "upper Chinatown;" some may be open as late as 10 pm. This article about Chinatown produce is highly recommended, btw: https://www.saveur.com/chinatown-produce-prices