When is the best time to see Greece ?

Mid May is warm but not hot. Late September is cooling down so warm days with a jacket possibly required in the evening.

Climate varies quite a bit across Greece, so it is somewhat dependent on what types of things you want to do and what areas you are looking at going to.

I believe the RS Tour stays mostly around Athens and the Peloponnese. For moderate weather, you would probably do best from early April to mid-June, and then September into mid October. July and August are reliably hot, and the islands and coastal areas are crowded. However, if you want to go North, to the Meteora and areas around there, even late June is not too bad

If time in the islands is wanted, and beach time is a big draw, I would opt for September over May. The water is warmer, and crowds have eased. May, the water is still a bit chilly, but if you are used to West Coast or beaches in the Northeast, probably fine for you.

If Crete is your interest, then you could probably push into March, and into November on the other end.

These days however, it seems as though a heat wave can strike at any time, followed by a snowstorm. As somebody once told me, the actual is likely anything but the average.

We have been to Greece twice the last week of May and first week of June. In both cases we went to islands first which were great at that time-pleasantly warm. However, both times we hit high 90s the first week of June on the mainland (Nafplio, Meteora, and Athens).

So if you don’t want hot, I wouldn’t go in June either.

I would also look at rainfall averages. Personally, I would rather be hot than wet but you might not feel the same way. We had no rain on our first trip and only last night on second.

My first reaction was anytime you can go! If that is not an issue and you can go anytime, I would suggest October. We still found it very pleasantly warm, but not roasting. We took the very last RS Greece tour that was offered, October 4th.

We visited Greece this past May for 2.5 weeks and the weather was perfect. I think our dates were around May 12 to May 29. We visited Santorini, Naxos, Nafplion and Athens. Santorini was cool at night but very comfortable during the day. Each destination was a little bit warmer as we got towards the end of May. Athens was the warmest; temps around 90 a few days. The other destinations were in the 70's to 80's. Very comfortable. We took a dip in the Mediterranean a few times. We did need a lightweight jacket or sweater for a few evenings in Santorini.

The advantage of going in the spring is that Greece is greener with more flowers.

April will be on the cooler side but you certainly won't need winter jackets. The islands will be cooler for sure.

Is May an option? I found late May absolutely perfect in terms of crowds and weather. We even swam a couple times, but it was not super hot, nor did we have much rain.

As people said, it depends where you go. For me, I've gone:
May: found Athens to be fine, but Meteora cold and rainy.
July: found everywhere pleasant, but sea not very warm yet to swim in.

I've told myself that next time I go it will be September or October.