Estonian weather is quite tricky... It can be in winter time around -30C and half a year later there will be +30C. Usually Estonians have joke that there are 10 month bad skiing weather and 2 month summer...

Best time of course to come is May-September. Then it is quite warm (but still take your warm jacket - you never know). If you like skiiing then best time for coming is January - February.

On March-April it can happen that during day there is -5 - +10 C in same day - just early mornings are really icy but afternoon if sun is high it is already really nice and warm...