(Almost) nobody likes to get married in Paris in the Summer
The graph below shows how many marriages there have been in France in 2008, depending on the size of the municipality -- rural, small town, bigger town, even bigger, and Paris and its surroundings. Base 100 is the average for the darkest and coldest months of the year, January to March.
What is striking is that from October to April the pattern of weddings is pretty much similar in all municipalities, but that starting in May, the small towns see a disproportionate amount of weddings when everybody flees for a nice sunny week-end in the countryside, increasing the chances that your guests will actually come to your wedding. There are more than 10 times more weddings in August in the rural counties, while for Paris the number only doubles between bleakwinter and nicesummer. Overall, one third of the weddings are in July and August, and while there are as many weddings in the smallest towns and in Paris in January to March, there are 4 times more weddings in July and August.
The smaller the town, the larger the number of weddings in every month from May to September; the difference between the different city sizes is most acute in August. So you might run into trouble finding a place to host your wedding party in the countryside, not in Paris, in July and August. June is very bad in Paris.
(disclaimer: I got married in Paris... in August - but the party was in September)
Number of weddings in France in 2008
These statistics are taken from INSEE - Données détaillées des statistiques d'état civil sur les mariages en 2008