Surprise vacation

I would like some tips for how to tell my partner I'm taking him to Cuba for new years. He's talked about wanting to go before it gets too "full of Americans/Americanized" and I agree, I put my points to good use and booked us 7 days in Havana at an adults only all inclusive resort, I've talked to his boss and got the extra days booked off

. It seems perfect, should I wait until Christmas? Tell him on his birthday in late September? Has anyone done this with success and have tips? It's so exciting and something we've both wanted for a while, I just want to make sure I'm not missing something.