Hello I need to find out asap what days and hours of operation is the Mae Sai border crossing open.

Open every day of the year, including holidays.

Not sure about opening times, suspect something from 8:00 till 22:00. probably open earlier, but not sure about immigration services provided.

Edit:

See post below.

Unless there has been a recent change, the border opens at 6am and closes at 5pm for farangs and 6.30pm for thais.