If you’re hoping to get lost in the rain on your romantic getaway, check out these details about year-round temperatures in Casselberry: The hottest months are usually July and August with an average temp of 82°F, while the coldest months are January and February with an average of 64°F. The rainiest months in Casselberry are August, September, July, and June, with each month seeing an average of 7 inches of rainfall.