How did the sumerians make sure that their calendar did not fall behind the seasons

Sumerian astronomers tracked movements of the stars, planets and the

Moon. Based on their observations, they invented a calendar which had 12 months and each month had around 30 days. Therefore, altogether, 12 months had 360 days. This was the first concept of  years, months and days.        

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Explanation: SUMERIAN CALENDAR

The days of a month would start and end at sunset. Each day had 12 hours, 6 hours of daylight and 6 hours of night. However, this calendar was not always completely accurate and every 4 years or so, the Sumerians inserted a extra month to the calendar.

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