If your system was installed properly there should be an automatic line drain at the low point - it's just a self-operating valve of sorts that closes under pressure, and then opens to drain into a buried gravel pit when pressure is off. The manifold and valves should also be below frost level for your area and again if installed correctly the supply line that goes to the manifold should also be below the frost line. Somewhere before the manifold there should be a shut-off, usually a 2" piece of PVC pipe sticking up that you stick a valve key into to turn off the water to the manifold and you should do that. So other than that and unplugging the timer you shouldn't have anything to do.
Of course, if it's not properly installed, or above ground, or some kind of strange thing like that then....
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