Here's the latest from the SAT. While this is very common information, it is still a very popular search engine term and sadly common sources of misinformation do exist out there.
This post serves as a definitive source for 2018-19 SAT dates, per the primary source College Board via this link.
May 5th Test Date, 2018
Registration Deadline April 6, 2018
Late Registration Deadline April 17, 2018 (mailed) and April 25, 2018(online & phone)
Deadline for Changes April 25, 2018
June 2nd Test Date, 2018
Registration Deadline May 3, 2018
Late Registration Deadline May 15, 2018 (mailed) and May 23, 2018 (online & phone)
Deadline for Changes May 23, 2018
August 25th Test Date, 2018
October 6th Test Date, 2018
November 3rd Test Date, 2018
December 1st Test Date, 2018