A day should be fine but I would recommend that you dont use conditioner and only a little shampoo. Its all about crating a barrier, if you use products then they could stop the dye being so effective or it could come out patchy. I rinse my hair with water before and then just blow dry it, then dye. If i have washed my hair the day that i dye it.
oh ive done it loads of times before, but have always used one you use on wet hair! thanks, i have washed my hair and i will do it tomorrow, IF i can be bothered, i will probably end up doing it during the week, i have waist length hair and it takes forever to do it :-)