World of Warcraft, people are keen to get to level 90 as that's really when the game begins. Also most people have already got level 90 characters so they don't want to waste much time getting to level 90 again and again. We all know that leveling to level 90 can be done fast, but can it be done in a day? Well maybe, depending on your resources. Generally if you are quick you can do it in 2-3 days.
To get to level 90 in a day you will want to get someone to powerlevel you with RAF or powerlevel yourself. Here are a few points to keep in mind.
1) Recruit a Friend is essential. So make a RAF account and powerlevel yourself. You can get to level 80 in a matter of hours with RAF so that helps a lot.
2) Heirloom gear is important if you are not powerleveling and simply soloing all the way to level 90. These are quick to acquire now and you can farm enough Justice / Honor fast.
3) Also if you are leveling solo you will need an in-game guide. These usually cut leveling time in half at least, and most players can easily get to level 90 in 2/3 days using these guides, with heirlooms you could probably do it quicker.
In the end getting to level 90 in a day will probably be pushing it but good luck! With a perfect game plan and a good guide it is possible.
Leveling Site: http://www.wow4powerleveling.com/