As ancient Latvians were engaged mainly in growing crops, they used the Solar Year as the basis for their time-reckoning system. Latvian Dainas (LDs) contain clear evidence that the four main seasonal festivals, recognized as the Annual Festivals (Gadskārtas svinības), correspond to the astronomical solstices and equinoxes:
Winter Solstice – Ziemassvētki (Winter Festival),
Vernal Equinox – Lieldienas (Big Day),
Summer Solstice – Jāņi,
Autumnal Equinox – Miķeļi, Apjumības (Harvesting festival).
The four ecliptic points provided recognizable clues that could be observed in nature, thus laying the foundation for the division of the year into smaller units – laiks (time), the proper names of which were formed by adding a seasonal characteristic to the term laiks (see figure), e.g. Ziemas laiks (Winter-time), Siena laiks (Hay-time), etc. and savaite – a nine-day long period. By dividing the year in this manner, the ancient Latvian time-reckoning system established a Perpetual Calendar where a particular day of the savaite and the date it represented remained constant and unchanged. In the reconstruction of the ancient Latvian Calendar, the Summer Solstice is most useful because it coincides with the Summer Festival personified by Jānis (pl. Jāņi) – Dievadēls (Son of God) that is celebrated for one day only when the night is the shortest in the year. The Latvian festivals, which formed an integral part of the ancient time-reckoning system, are still known by their original names (Grīns, Grīna 1992).