Typical sprinkler controllers only turn one valve on at a time. Not only does it save current but often the zones are configured so that there's only enough water flow for one zone at a time.
You don't need a calendar, you need a photocell. Winter is easily distinguished from summer (assuming a latitude where there's a difference) by the difference in the average daylight times. Similarly, since cloudy (and rainy) weather should influence the watering schedule, the photocell will also help this. If you need a schedule with a weekly synchronization (i.e. never water on Sundays) then divide by seven. A thermistor and/or humidity sensor might help with the watering algorithm.
User would select increase/decrease per zone to compensate for water requirements.
Chip suggestions: ATTiny15, ATmega48/88, LPC2101.