Summer in Estonia tends to be hotter than you might think, due to the long hours of sunshine, and the warming effects of the Gulf Stream, and without a doubt, the best time to visit Estonia is during the high season that runs roughly from the middle of May through until the middle of October.
You do get some fantastic bargains on hotels and flights over the winter months, but the weather can be bitterly cold, and many of the tourist attractions do not open - although during the few hours of sunlight that you get every day, the countryside can be very picturesque, and there is some halfway decent skiing available.