Any time you can get on the soil without compacting it. Mine is too wet to work on at the moment - when it's not frozen, that is!

I would also suggest that chicken pellets are not necessarily the idea product to be putting on your veggie bed. Compost, well rotted manure, seaweed, anything that is going to add organic matter to your ground will work better. The pellets can actually burn some veggies.

Wouldn't they rot down before April, Mg?

If you do use chicken pellets they should be added at the time of planting or no more than a week before as they rapidly loose their effectiveness, unlike bulky organic matter which keeps feeding the soil for months. But, as I said before it can burn seedlings so you have to be careful where you use it.