Wednesday, February 12 is Buddhist holiday. Where can I find if this will result in closing of Chiang Mai immigration office?

August closures.

Please note that immigration offices around the country will be closed on August 1st and 2nd (Buddhist Lent day and Asalha Puja day) and also on August 14th (a substitute day for the Queens birthday on the 12th).

Plan accordingly.

Land, sea or air borders are open as normal.

Edit: It appears that July 31st is also a holiday too. This means immigration offices will be closed from the 28th July until the 2nd of August and will reopen on the 3rd August. Avoid the 3rd if you possibly can. Many offices will be super busy with that amount of time being closed.