In the dormant season, late autumn to mid winter.

Acer palmatum is best not pruned at all unless absolutely necessary - i.e. where it's outgrown it's position of is seriously encroaching on another plant.

The reason for this is that the newly cut/pruned branches are notoriously susceptible to fungal attacks and die-back which can, in some cases, end up killing the entire plant. Scrupulous cleanliness of your pruning implement is vital - I would sterilise with boiling water immediately before use.