For what I understand the system is smart enough to determine if the lawn requieres any water based on the location, humidity, temperature, season, etc. I always leave the system on year long and I don’t have to worry about it, during winter, several weeks passed and the system did not turn at all until recently. 

From the main page, just check the zone you want to water and click on the “play” button. That will water the zone for a few minutes.