The oft-leaked new 4-inch iPhone 5se has so far been rumored to be officially introduced at an event on March 15. On the same occasion, Apple will reportedly also announce the iPad Air 3 as well as certain improvements for the Apple Watch (but not a new wearable).
Now though a new rumor out of South Korea claims that Apple's big spring event has been pushed back - though just by exactly one week. Hence, it's apparently going to take place on March 22.
Unfortunately Apple itself hasn't yet sent out invites, so take all of this with a pinch of salt since it could be nothing more than speculation. The iPhone 5se should cost $450 with 16GB of storage, sans contract. Past leaks have said it would sport specs borrowed from the iPhone 6, but in a body the same size as that of the iPhone 5s.