ATM machines?

There are 2 ATMs at the airport, use the Banamex one - not the Bital (adds a fee, the only bank ATM that does here). You must have a four digit PIN. The bank at the airport gives the best exchange rate in town.

Is the bank at the airport open on saturdays ?

What about Sundays?

Yes the exchange at the airport is open on Sundays. Not sure of the exact hours, but do know it is open at 3:30pm when we arrive. This is where you get the best exchange rate. Once in town, ATMs are reasonably easy to find.