Great job overwintering your dahlia tubers. The time to replant them outdoors is when all danger of frost has passed and the soil has warmed. In most areas this would be sometime in May.  Every few years you will want to divide the tubers by cutting them into smaller chunks before planting. The tubers branch outwards like finger  on a hand. Try to separate these fingers so that each chunk or section of tuber has a growth eye or bud similar looking to an eye on a potato.  To replant loosen up the soil in a sunny site and bury the tubers so that there is 6 to 8 inches of soil on top. I like to put a wire tomato cage over the area after planting so I won't accidentally dig into the spot. If slugs are an issue in your area be sure to bait once the new green growth pops up, Be sure to mix in some Osmocote plant food near the tuber when planting. Dahlias like plenty of fertilizer to produce their abundance of beautiful blooms.   Keep growing, Marianne Binetti