The CW released its spring schedule including the date for the season finale for Supernatural.
The season finale of SUPERNATURAL will air Thursday, April 25 (8:00-9:00pm ET/PT).
Also, carrying forward the current schedule of aired episodes, the 300th episode, which will the episode 13 of the season, should air on February 7, 2019. 
2019 early winter
Will keep you up to date as we get more information.