The season for E-filing tax returns officially ends on October 15th. You can still mail in your 2009 tax return after the deadline but your return will be considered late. Remember that all returns mailed after the deadline must be signed.
Tax returns filed prior to the closing of the tax season must have their postage affixed and be postmarked by 11:59PM on October 15, 2010 in order to avoid late filing penalties. If you have a liability for tax year 2009 and you do not file by the deadline, penalties will be added to the amount owed and interest will accrue from the date of the original April deadline!