FunClucks
Crowing
The younger the better, since she'll think they've just hatched from an egg. I'd go Thursday. There's quite a difference in behavior in 3 day old chicks (what you'll get from the feed store after they are shipped to the store) and 6-7 day old chicks (what 3-4 day old feedstore chicks actually are when you count the number of days from hatch). Make sure the chicks have eaten and drunk well, then slip them under momma. Especially remove the eggs as soon as you put the chicks under - that way she might go get them something to eat sooner since she thinks she's done. Normally hens will sit on the nest for 3 days after the first chick hatches waiting for the others to hatch, and the chicks survive on the yolk sack during this time. The chicks from the feed store have already used the yolk sack up, so will need food the very next day if you slip them under her at night.That’s actually a great idea. This poor girl was broody from April- august last year and I was so excited to get her some babies. There is a feed shop with day old chicks and I can go Thursday or I could get 3-4 day old today… I’m guessing waiting until Thursday is better?
If/when she goes broody again and you don't want chicks, please break her as soon as you notice, if you're able to. It's not healthy for her to be broody for long periods of time. Here's a great article that helped me.
https://www.backyardchickens.com/articles/broody-breaking-ala-aart.77915/