Summer in Europe July (hopefully.
The end of April was nice and warm in the UK, certainly where I live central heating off; then bang all this week has been fairly cold temps below zero on two nights close to zero on another. There was me thinking this global warming is nice warm weather in April. Hopefully Summer will be warm and last more than a week
So Summer is perhaps not a good term to use for a global product, so a best guess would be July/August and fingers crossed.
One thing for sure you will hear about it on the forums first (beta release, etc.), a little before any planned release.