Update: Over the weekend, Re/code reported that this event will actually be held on "the week of March 21", not March 15th as previously reported by other outlets. The content of the announcement, which will reportedly be held in Apple's small on-campus Town Hall meeting space, should be the same as outlined below. Official event invites typically go out a couple of weeks before the events, so in any case we'll know for certain soon enough.
Original story: Mark your calendars: A new report from Buzzfeed's John Paczkowski says that Apple is planning a product event for March 15, its first since the iPhone 6S, iPad Pro, and Apple TV event it held back in September. Paczkowski's sources have reliably predicted the dates of multiple iPhone and iPad events in the past, and 9to5Mac's sources have also pegged March 15th as the event date.
Apple's plans could change, but as of this writing, Apple is expected to make a couple of (literally) small announcements. The most significant is the "iPhone 5SE," a 4-inch iPhone that combines the approximate size of the iPhone 5S with many features from the iPhone 6 and 6S. Current rumors say that Apple Pay support, an Apple A8 or A9, upgraded LTE, and a lightly modified design will be the most significant features and that the phone will replace the iPhone 5S at the bottom of Apple's iPhone lineup.