Doing this allows the natural oils in your hair to develop which in turn allows the dye to bind to your hair more easily. The dye will blend more naturally with your hair, and that tends to make it last longer.  If possible, avoid conditioner when you wash your hair the day before dyeing it. Conditioner eliminates your natural oils, which you need for the dye to set in more easily. If your hair is extremely dry, condition it every night for at least five minutes in a hot shower for a week before you plan to dye your hair. Then, do not condition your hair the night before dyeing. This will keep your hair from drying out after you dye it. It’s easy to get overwhelmed by the hundreds of shades you can choose from. If this is your first time dyeing your hair, its best to stick to a hair dye that is no more than two shades darker or lighter than your own natural hair color.  If you are new to dyeing, you can also try dyeing your hair with a temporary or semi-permanent dye first. Using one of these types of dyes ensure that if you mess up, you won’t have live with your mistake for long. Keep in mind that you have to apply semi-permanent dye to damp hair. Temporary dyes, such as chalk and mousse, usually wash out after 1 to 2 shampoos. Semi-permanent dyes will rinse out after 20 to 26 shampoos. Permanent dyes are permanent. The color may fade after 6-8 weeks, but will need to grow out.

Summary:
Wash your hair 24 to 48 hours before dyeing. Pick a color that you really like.