It is unknown how dragons keep time - Bethesda certainly did not provide any info. The following are Legacy vocabulary.

January - Feylil - Morning Star
February - Kreinvu - Sun's Dawn
March - Diistmaas - First Seed
April - Haalokluv - Rain's Hand
May - Ziistmaas - Second Seed
June - Mitheruvos - Mid-Year
July - Shulnaar - Sun's Peak
August - Laatmaas - Last Seed
September - Silyol - Soul-Fire
October - Fomah - Frostfall
November - Kreintor - Sun's Dusk
December - Vulonfil - Evening Star