The extension deadline for filing Form 990 series returns for tax-exempt organizations is Monday, August 15th. This deadline is for the 501(c) series of organizations that filed extensions back in May, including charitable organizations, welfare organizations, social clubs, and other nonprofits. Most of these entities will not owe any tax unless they have either unrelated taxable business income or are a private foundation with investment income. If filing a paper return I strongly recommend using certified mail, return receipt requested so you have proof of filing. Better yet, just file electronically and you have complete proof of filing.