First sunday of the month? Hope for sunshine. Lots of it.
All public museums (namely 90% of them) are free today.
The more sunshine, the more people will not queue, the more you can take advantage of this.
Get up early. Specially if you’re aiming for the big ones. For Louvre, for instance, you should be there at 8h45. Orsay as well. The other ones, it won’t be that bad.