Question
I have a weeping maple in my front yard that has gotten very wide and I would like to trim it.  Can I do it now or do I need to do it at a certain time of year?

Answer
The simple answer Is you can do it now.

But, how much are you removing?

1) never remove more than 25% of foliage in a single year, regardless of when you prune
2) if removing more than a very small amount, it is preferable to wait - the tree has exerted energy to produce foliage, let that foliage return energy by photosynthesis before removal is always preferable ( in other words once leaves are on, let them do some work and gain back some energy the tree used to produce them )

If your are removing larger volume of foliage in total, say close to the 20-25% range, remove small amount now, and some more in a few weeks.  This give you some relief now, and allows the tree to get some benefit as well.  A compromise we will say.