When traveling to Tallinn, it’s pretty much guaranteed that no matter what time of year the trip takes place, it is not going to be hot there. Tallinn is definitely not known for having hot summers. Summer, however, is the most common time of year for tourists to visit. The average temperature during the summer months (May through September) is about 70 degrees Fahrenheit. The temperature can climb into the mid-70s, but usually no higher.
During the summer, even with temperatures in the 70s during the day, the nights can be quite chilly, reaching into the 40s. So, tourists who travel to Tallinn in the summer should make sure to pack a jacket, a sweater and anything else to keep warm at night.
The winter months of January and February are the coldest months that Tallinn sees. The average temperature during the daytime is usually around 20 degrees Fahrenheit, but it can go as low as 20 below zero at night.
Tallinn and Estonia see a fair amount of rain and precipitation during the year, and this is one reason the country has such a good agricultural industry. However, for tourists, it can mean enduring some very cool rain showers during a visit.
Don't write off a winter break in Tallinn. However, it would be better to go when it's coldest ie. with the greatest chance of snow, since this would see Tallinn at its most magical. But note that while November-December would be milder, it may well be raining, and so long as you're prepared (long Johns, fur hat etc) the weather being cold is better than it being wet.