Trimming ash tree
Thank you for asking this question.
The best time to prune an ash tree is in the fall.
See page 12 here. http://www.mntreesource.com/uploads/2/0/7/0/20706756/updated_citizen_pruner_manual_rochester.pdf
All of the tree wood from your ash must be disposed of properly. New rules are being developed for Stearns County: https://www.mda.state.mn.us/emerald-ash-borer-discovered-stearns-county They will mirror these, https://www.mda.state.mn.us/plants-insects/ash-tree-waste-disposal-sites Contact your County Department of Agriculture to learn the procedures.
All in all, this is a job for your arborist; it requires much more expertise and special resources than pruning other shrubs or trees.
Other resources suggest best time to prune an ash is late dormant season - just before Spring. Why do you recommend Fall?
The general rule for pruning trees is to do the dormant season. With regard to ash,
and diseased branches should be pruned anytime in the winter to late winter. Pruning for shaping is ok in the fall after the flight season ends--generally in August. http://nyis.info/eabbiology/
Spring is earlier and earlier because of warming. In the past, borer was active in May but it is possible for it to be earlier. To not get into the time of active infestation of living branches, that is for shaping, fall seems safer. Dead or clearly diseased branches can be taken down anytime. All pruned wood should be treated as infected.
I hope this helps