How to prune and trim versus the crepe murder can be a spirited discussion. Many commercial landscapers routinely do that severe cutting back of the tree. Many folks in the garden world -don’t understand this.
Here’s a few helpful links to help you with guidelines for proper pruning.
Hope this helps.
Kathy, it depends on what zone you live in, so I can't give you a definite answer. Here are several videos that may correspond with the area you are in. I hope this helps you.
You can prune crepe myrtle anytime before it begins to grow leaves. It flowers on new wood (new growth) so pruning in winter is fine.