As a professional preparer, you may electronically file returns for the current tax year as well as the previous two tax years. For example, during the 2018 filing season you may file 2016, 2017 and 2018 returns.
Note: The IRS shuts down its Modernized e-File Program (MeF) system briefly (sometime in November for 1040 returns, and near the end of December for business returns), in order to prepare for the next filing season. The IRS begins accepting returns again around the middle of January. If you need to e-file during this shutdown period, or if the deadline to e-file your return has passed, you will need to paper file your return.