Calais has a maritime climate — mild winters, rarely hot summers, and a tendency to be overcast and windy in any season. The best weather is between May and September, with July and August being the warmest months, though temperatures rarely exceed 25°C. The ferry crossing can be rough in autumn and winter, though modern ferries cope well enough.
If you are crossing from England for a day trip, any weekday outside summer school holidays will be quieter. The Saturday market on the Place d'Armes is a good reason to time a visit for a weekend morning.