The decision is yours, but when my x and I decided to divorce (he didn't want to and I couldn't stay another minute) I told the kids immediatly even though it was the begining of November and my daughters birthday is always the same week as Thanksgiving. I moved out, with the kids, the weekend after Thanksgiving. My daughter was 8 and my son 10 at the time.
Fast forward about 4 years, having always felt guilty for leaving during the middle of Holiday season and my daughter, completely out of the blue, tells me that she used to pray for us to get divorced. She doesn't remember what time of year it was, that the holidays were close or that her birthday was near....she only remembers the relief that she wouldn't have to listen to mom and dad fight anymore.
Both my kids, on seperate and unrelated occasions, told me that they used to pray that we would get divorced. They've also told me that things were more difficult than they thought they would be after we moved...(they had to make new friends, not having us in the same home)...they were still happier living with us individually than together.
Kids know way more than we give them credit for but don't have the skills to deal with divorce itself and will need quite a bit of guidance. In my state a 'children in the middle' class is required for divorcing parents...it helped me help my kids tremendously! I would advise that you take such a class or make it a part of your divorce agreement that both parents will have to take it...if it isn't required in your state. The class goes through the ages and stages of kids and what to expect, what to look for as signs that they are having trouble adjusting. With this knowledge in hand, I was able to help my kids, and continue to help my kids...deal with mom and dad not being together anymore and the problems that inevitably come up. Good luck to ya hun, you have a hard road ahead of you, but with the right attitude (kids first) and armed with some skill and knowledge...your family will survive no matter when you choose to tell them. (My kids are now 14 and 17 and doing great!) Hugs...