They may be open but when you take the modem make sure you take the power supply as well. I did that last week only to find out that my power supply was at fault and I happened to have an extra one here, I guess they overheat every once in a while. If it is bad they will replace it gratis!
Telmex just called us to say they had just repaired our internet line about an hour ago, so I know at least the repair department is open. You might try them; they just may have shorter hours Saturday.
-Gail