mister, I've been in April (Cayo Guillermo), May (Guardalavaca) and November (Cayo Santa Maria)
Guardalavaca in May was quite hot (but I like it hot,lol)
Cayo Guillermo in April was perfect
Cayo Santa Maria in November was nice in the day, a bit cooler in the evening though I never wore a jacket, but some people were quite chilly
Plus the water was much rougher and lots of sea grass at that time of year. Red flag days everyday except for the last full day there
When I came back in November, it took me a full week before I felt warm again (I came back to a cold spell, brrrr)
I don't like going months before we have nice weather here in canada again
I think late spring and early fall are perfect times to go. In the spring you come back to canada and the weather is getting nice here, if you go in September or early October, you also usually still come home to some nice fall weather. And it gives you time to aclimatize for winter :)