We have seen a couple of reports saying that the Galaxy S8 (and the S8 Plus) could go on sale on April 21st, and according to serial leakster evleaks, Samsung could push the release of its next flagship by a week. Perhaps it shouldn't come as a surprise that it could take longer before we are able to get our hands on the Galaxy S8, as Samsung would want to make sure that the Galaxy Note 7 debacle isn't repeated.
Furthermore, since no official dates have been announced yet, there is no telling whether any of the reported release dates will actually turn out to be true. For all we know, Samsung could put the Galaxy S8 on sale before April 21st, given that the phone's official announcement will be happening a month later than previous Galaxy S flagships. It also remains to be seen if the previously rumored April 10 pre-order date will be affected by the delay in the phone's release.
Until something truly concrete comes knocking (you know, like a leak from a carrier), we would recommend taking everything with a pinch of salt. The Galaxy S8 will be unveiled on March 29th at dedicated events in New York and London.