The night sky is divided into 88 constellations. Taken all together, these constellations are called the celestial sphere. An observer can only see a portion of the celestial sphere depending on their location on Earth and the time of year. By observing which constellations are visible, you can figure out where you are on Earth and what time of year it is.
On the night of a full moon, you can draw a ray originating from the sun that passes through the earth, and continues through the moon. This ray will point to one of 12 constellations. This constellation gives an astrological "month" of the year.
Our ancestors used the location of a full moon to know when to plant and harvest, but today, we just refer to these markers as seasons. The constellation that is closest to the full moon's position gives the season. Here is a chart for reference: