In winter, the average temperature is 14 degrees and very rainy, in summer the humidity is very high and the air temperature is around 30 degrees and is quite dry.
The holiday on the island of Rhodes begins in April and lasts until November. The hottest months start in mid-June and last until the second week of September. Therefore, if you do not want to be affected by the heat, April, October, and November will be more ideal.
Winter Holidays in Rhodes Island
If I were coming to Greece in the winter season and had to choose one island to visit then this would be the one, though I would probably fly rather then take the ferry, to avoid storms and sea-sickness. As for other times of year we were there at the hottest time of the summer and it was fine.
Even the hottest part of the day you could find somewhere to stay cool, either by the sea, in the shade in one of the many parks or in a cafe. Even the hottest days the mornings and nights were fine. When the summer meltemi winds are blowing the climate is perfect. I can’t imagine it being any better in April, May, June, September and October but they say it is.
Keep in mind that there are fewer island connections in the winter and getting from Rhodes to Santorini and the other Cyclades is much more difficult in the winter. There is a ferry to Crete which continues on to Mylos and Santorini a couple times a week but this is a long trip, especially in the winter when the open sea can be rough.