It depends on which type of hydrangea you have.
Lacecap macrophylla, serrata, aspera, climbing - they develop flower buds for Spring 2021 in late Summer/early Fall 2020 (the flower buds are "invisible", located at the ends of the stems) so, wait until after blooming to prune but feel free to deadhead spent blooms at any time.
Lacecap arborescens - they develop flower buds for Spring 2021 in early Spring 2021 (the flower buds are "invisible", located at the ends of the stems) so, wait feel free to deadhead spent blooms at any time and you can prune any time after blooming but before Spring starts.
* you can prune any stems that do not leaf out by the end of May 2021 all the way down.
* if planted in a location where they can attain their estimated size on the plant label, you should not need to prune for pruning sake.
Viburnum Snowball should also be treated as a lacecap macrophylla as it blooms on old wood too. Prune it after it blooms in Spring, not now.
