From mid May to begin June
Japanese hollies can also be pruned as needed in midsummer or late winter. If pruning for hedges, late spring is a good time for trimming holly bushes.
Read more at Gardening Know How: Trimming Holly Bushes – How To Prune Holly Bushes https://www.gardeningknowhow.com/ornamental/shrubs/holly/trimming-holly-bushes-how-to-prune-holly-bushes.htm
- From mid May to begin June Once