There's 4 eps left so it's probably going to happen on May 23rd.
Edit: Yup.
http://wwlp.com/2017/03/13/season-finale-dates-for-may/
DC’S LEGENDS OF TOMORROW – Tuesday, April 4 at 8pm
SUPERGIRL – Monday, May 22 at 8pm
THE FLASH – Tuesday, May 23 at 8pm
ARROW – Wednesday, May 24 at 8pm