Cuba has all the year round hot weather. May should be pretty hot. Cayo Guillermo should have a temperature of around 28-30°. It will cool down in the evenings but the temp. will still be high. No need for a cardigan unless you intend staying indoors in air conditioned areas. I would think jeans and a jacket would be too warm.
You may get some rain in May though.

Hi, should be good, a light long sleeved cover-up at night if you really feel the cold...try this link:

http://www.holiday-weather.com/cayo_gilluermo/index.html#weather-overview

ATB,J.

Why not travel wearing jeans and several layers on top, that way you have something heavier should you find it cold in the evening (and you can always shed the top layers when you get there!). If the sun is out it will certainly be hot.

Damn, I´m really jealous!!

I can't imagine anybody from the UK finding Cuba in May cold! Havana has a reputation amongst Cubans of getting cold - I mean in the winter the temperature sometimes drops as low as 22C! I would be very surprised to find that you needed a jacket. My advice would be to leave the jeans at home - I've been in both March and July and would have found jeans much too hot at either time. if you are spending time in an air conditioned environment then it can be useful to have a longsleeved shirt or T shirt to pop on but I can't imagine that you'd need anything heavier. I find that carrying a lightweight pashmina in my bag is enough to combat the sometimes arctic temperatures on air-conditioned Cuban coaches but that's about all.

SM