Ok, I'm trying to remember over 15 years worth of traveling to STJ (I can't help it; I LOVE it there!). I've been in January, March, April, May, June, July, September, October, November. Would have been easier to say I've never been in December, February & August
My favorites and why--
January: on-season rates but so worth it when I leave my home in temperatures that are -24 below with snow & slush and VOILA! 12 hours later I walk down the plane steps & feel the wonderful hot air breezes brushing my face which is now miraculously converted from being framed by a fine-hair wisp to BIG HAIR!

Plus being able to be on the beach & swim in that beautiful crystalline water while at home everyone is standing at the gas pumps dressed up like abominable snowmen & shivering in the sleet & cold. Like cheating 'mother-nature.' I LOVE it! The previous is DITTO for ALL on-season travel!
March & April: All of the above plus being able to get an early start on my tan, plus being tired of waiting for it to be SPRING which never really arrives in the Great Lakes area until May (after 5 LONG months of yukkie winter weather), plus in earlier years being able to spend time with my kids on spring break from high school or college.
May & June: Summer's arrival at home is still pretty unpredictable so ditto the above early March & April comments plus, I forgot to mention above, that ALL the great restaurants and shops are open from Christmas through end of July. I have developed
addictions to Lime Inn lobster, Tage & La Tapa ANYTHING, Ronnie's pizza, the Balcony's lobster ravioli, Cafe Roma, Joe's BBQ, Miss Lucy's eating a greek salad surrounded by goats & baby chicks, etc. Plus usually zero humidity, great warm weather & water, nice breezes so no mosquitos. Love it!
July: one of our favorite places to stay (all of our favorite places to stay) have sunset views which means, on July 4th, they have FIREWORKS views! (I'm still a kid at heart: I LOVE fireworks, Christmas trees, & elephant ears at fairs). Ditto most of the above June & May comments but it's starting to get quieter (AFTER the 4th) but most things are still open.
September, October & November: I LOVE these months because I feel like a local & can pretend I actually LIVE here! Easy to find drop-dead beaches that we have all to ourselves, roads are not crowded at all, local people smile at you and are happy that you are here, water is the same temperature as the air--HOT--which is what I LOVE (I must be an amphibian at heart!). Lower villa & car rental rates. About 1/3 of the restaurants (if not more) are closed but the ones that are open are full of locals & happy travelers & people who are very welcoming and glad that you are there. VERY calm seas (unless of course there's a hurricane which when there isn't we've instead been blessed with absolutely awesome weather).
Ok, after MANY words I have to concur with an earlier poster who basically said the best time to be in St. John is when you are there!
