The deadline for federal tax returns is April 17, 2007. This is due to the celebration in the District of Columbia of Emancipation Day (the anniversary of the Emancipation Proclaimation), and the celebration in some Northeast states of Patriot's Day. Colorado's returns, and returns in most U.S. states are also due on April 17, 2007.
In particular Massachusetts, Michigan, New Mexico and Ohio (I know I have raders in all of those states) are among those states with an April 17, 2007 state tax deadline. If you aren't in one of those states, check on your local deadlines before risking filing late.