Turning serial connection on and off to save batttery, possible?

I have a GSM800 module that is only to send one sms per day. It has its own battery and is turned on and off by mosfet and a output pin.

I am reading you don't get a Serial port if nothing is connected at startup to D29/29 on the Puck..Does this mean the serial port must stay active using the oscillator the whole day, or can I turn it on and off at will?

Thanks