I don't think you will find that answer, "when will they open the position of Flight Attendant" Here are some things you can do?
The last opening for a Flight Attendant job was 2/5/18 and now is closed.
"In order to remain consistent in our hiring practices and to be fair to all applicants, they only accept applications for a specific position, and only when the job listing is active on the Careers page."
The flight attendant positions would be listed under the city of Dallas Texas, the Southwest Airlines main headquarters.
The best thing you can do is set up a profile to be notified when jobs become available here https://tinyurl.com/y8p82yya
See current job openings at Southwest Airlines here Careers page https://tinyurl.com/yalmfmzr
Good Luck!
