Hi Lizz66...
Having a French hubby, we have been invited to a few weddings over the years, and to sum it up, your "toilette" absolutely depends on what region the wedding is being held in ... the Paris region, for example, is notably smarter overall. It also depends, as in any other country, the "social" (perceived or otherwise!) standing of the family involved. The age(s) of the couple also play a large role in the overall tone of the day, again like anywhere else. To give you an idea, I have seen attendees dressed in the full monty chatting beside someone wearing jeans! This is the biggest difference for me compared to the North American, Asian or British weddings I have been to.
Being very attached to my scottish roots, and loving the fact that I am not someone who conforms very well, I
always wear a hat for the daytime festivities -

- and while there have been only one or two other women at a couple of the weddings wearing one as well, I fear I am known as "la folle Ecossaise", a title I bear proudly ...

One thing I have noticed though, is that often there is a break between the "vin d'honneur" and the dinner in the evening (if you are invited for that). It is very common for the women, and sometimes the men as well, to change outfits ... of course this again depends on the weather and formality of the event.
Regardless, you will have a lot of fun and a lot to eat and drink!!
Cheers
Fiona