Pension plans covered under ERISA section 4021 must annually file premium information with the Pension Benefit Guaranty Corporation (PBGC) and pay the premium due, if owed. Filing due dates differ based on the plan size. For large plans (those with 500 or more participants), the filing is due the last day of the second full calendar month of the plan year, which is February 28, 2011, for calendar year plans.