In addition to what everyone else has said, the weather may be a consideration. If it's cold and rainy it can be really slippery which can slow you down (or speed you up I guess depending upon how strong the rain/wind/snow is). Also - my experience with the first few days of a camino is that they're harder than it is after the first week. Having said that, although I stopped at MOnbonnet, it's certainly possible even for someone who's older than dirt (like me) to make it to St. Privat the first day. If you have a time shortage maybe it's a good idea. Otherwise why rush.
And, as others have said, looking ahead, the climb out of Monistrol may be quite a challenge at the end of a day. And it's beautiful - you want to see it!!
Don't forget to always have snacks with you in France. The stores are always closed on Sundays and sometimes Mondays - or Thursdays - or maybe Tuesdays..........