Yes, you can mail anytime before April 17th, but I would highly suggest you mail your return via certified mail. Anytime you mail any documents to the IRS (or any federal agency), you want to have an official record of your mailing and to ensure that it was received and acknowledged - its well worth the extra money if there was ever an issue with the receipt of your documents (especially when you owe money as the penalties for not filing and paying can apply).