Form 5500 has to be filed yearly with the the Employee Benefits Security Administration (EBSA), at the Department of Labor, by any organization with an employee retirement plan (small orgs can file Form 5500-SF). This reporting is mandated by the Employee Retirement Income Security Act (ERISA). You may file online using this thing called EFAST, for which you have to register. Handily, if you have multiple clients, you can do all of them under your own registration.
Note that many employers offer two plans—a defined contribution (DC) plan and a tax deferred annuity (TDA). You must file a 5500 for both, even if one is inactive.
For a plan year that corresponds to the calendar year, the form is due on July 31. Otherwise (and similarly), it’s due “by the last day of the 7th calendar month after the end of the plan or GIA year (not to exceed 12 months in length) that began in” the year you are filing for. (A GIA is a group insurance arrangement.) The confusing thing about this is that, unlike a fiscal year, a plan year is named for the year in which it begins. In other words, if your retirement plan year runs from June 1st, 2016, through May 31, 2017, you file a 5500 that says 2016 on it.
You can get a two-and-a-half-month extension (to “the fifteenth day of the third month after the normal due date,” i.e. through October 15). To request an extension, file Form 5558. You’ll need to file a separate request for each plan you’re reporting. This has to be done by mail, but the PDF is writeable.