One member who's name escapes me now has successfully done 3 days before and 3 after a relaxer. I also tried her method with no adverse response. Just beautifully strong, straight hair, with little shedding.
I woudl wait until about 2 weeks after. I actually havent's relaxed yet since I've been hennaing. I have noticed that henna has changed my texture (loosened it) a bit.
I say wait two weeks because for me it would take ALL DAY to relax and henna!!