After the Edain had settled in Númenor, they set about creating a new calendar for themselves. Though their ancestors in Middle-earth had followed a tradition that the year had begun in midwinter, beyond this they had not possessed any kind of formal calendar of their own.

The early Númenóreans used the calendars of the Elves as a basis for their work, but the detailed operation of these calendars, designed for the long-lived Eldar, required revision to suit the needs of mortal Men. Where the Elves divided the year into six long 'seasons', the new 'King's Reckoning' of Númenor broke the year into twelve shorter months, which were known as astar.

Each asta, as far as possible, was thirty days long. This could not be achieved perfectly, and so two longer astar of thirty-one days each, Nárië and Cermië, were applied either side of Mid-year's Day. This arrangement lasted through the entire Second Age, and long into the Third, until it was further adjusted by Steward Mardil Voronwë. Mardil's 'Stewards' Reckoning' equalised the lengths of all the astar at thirty days each. He achieved this by creating two festival days that were not included within any of the twelve astar: Tuilérë in spring, and Yáviérë in autumn.