When is the best time to stain a deck?

You need a few straight days of moderate temperatures with no rain in the forecast. Depending where you live, wait until all chances of frost have passed so it doesn't mess up your stain.

Deck stain timing conditions would be listed on the product label.

Behr and Valspar have excellent customer support if any question or problems, and have provided replacement product when a product failure has occurred.

Typically you don’t stain a newly constructed deck immediately and each product has application guidelines including the environmental temperature parameters. Proper prep is needed before applying product. Decide on which product to use and check your label for pretreatment cleaning and temperature requirements.

Just depends where you live and the humidity,moisture,rain,snow in area.

Dry in the sun

This may help: https://www.thespruce.com/how-to-stain-a-wood-deck-1825147