Deauville is a destination favored by Parisiennes in their Summer holidays. So if you are planning in traveling in the month of July and August expect lots of vacationers everywhere. If you want to travel around, roads will be messy specially in the weekends in August until the first week of September, try to plan your day trips around those two busy days.

On the other hand, Deauville main events and festivals (the American Film Festival, main horce races, auctions) take place in those months, that is something that you need to take into account when deciding when to go. If you are into horse racing, you can probably go in other time as the season goes beyond those two months.

Summer will give you the opportunity to enjoy the beaches, though, and depending on the activities that you have in mind, in Fall and Spring the beaches have a charm given by the peace and the grey tint of the landscape. Also, if you are visiting the museums in town and the ones near Caen and Bayeaux after September, you'll find them much less crowded giving you enough time to tour the sites. The downside is that because there are less tourists, some of these sites are open half day or they close earlier. Check their hours of operation before visiting them.