They are open, but the officer who signs these applications may not be in on Saturdays.
I went there once on a Saturday trying to change my British driving license to a Qatari one and they couldn't authorise my application as he wasn't there.
They work Sunday - Thursday from early morning till 6:30pm (but if you want to go in the evening, trying to get there before 5:45 otherwise you might find that some of the workers there left early).

Hope that helps !