Most Holy Calendar of the Covenant

A Unit is the smallest measurement of time in the Covenant’s timekeeping system. Equivalent to 1 Earth hour.
A Cycle is composed of 265 Units, or roughly 11 Earth days.
A Quadrum is composed of 8 Cycles, or roughly 88 Earth days.
An Annum is composed of 4 Quadrums, or roughly 352 Earth Days
An ‘Age’ is an ill-defined method of Covenant historical recording, often without order and utilised to enact grand sweeping changes across Covenant society. Ages of Doubt were used to declare states of emergency, Ages of Reclamation were used to signify a discovery of religious import, and Ages of Conflict were used to deliver edicts for wartime.
Covenant contact with Humanity spurred them into an Age of Reclamation rather than an Age of Conflict due to Humanity being accused of destroying Forerunner relics en-masse. This would spur the Covenant into a religious pogrom of such scale and zeal that none would dare look closely at the reasonings behind the declaration lest they too be considered heretical.
Annums are reset following the beginning of each Age. So the end of the Human Covenant War came on Annum 30 of the Ninth Age of Reclamation, by Covenant reckoning.
Covenant Warriors and Citizens adhere to a 53 Unit work rotation. They are expected to work for 13 Units, tend to their spiritual needs for 10 Units, engage in ‘self-betterment’ for 10 Units, and finally sleep for 10 Units. This rather unforgiving cycle was made to benefit Sangheili and San’Shyuum physiology, and can be quite demanding on other races. 
Unggoy are often seen as slothful as they require far more than 10 Units of sleep, leading to them sleeping during their allotted spiritual or self-betterment hours, or even sleeping while on-duty.
Quadrums are named for the time of Cycle they resemble in relation to the start of the Annum, similar to how spring is associated with birth and renewal and winter is associated with endings in Human society. Celebrations are allotted for each to be observed during the mid-point of the Quadrum, IE, after 4 Cycles has passed.
These Quadrums are;
* The Quadrum of Toil
** This Quadrum is celebrated by awarding highly-skilled labourers with merits and a Parade of Labours observed on many worlds, which celebrates those that do important work for the Covenant that are not frontline soldiers. Artisans, weaponsmiths, armourers, and traders are given the majority of the Cycle off to be with their kin. A feast is held with priority given to those who are in gainful non-military employment, known as ‘The Fruit of Labours’ feast. Communities and Keeps come together with others to provide for the Feast, and enjoy it alongside one another. Assassination at the Feast, or in any of the Quadrum celebrations, is deemed cowardly.
* The Quadrum of Piety
** This Quadrum is marked with a period of spiritual self-reflection, and the awarding of accolades to the most pious voices among the Covenant. Smaller communities based on planets or in Keeps award the most dutiful and well-known voices. Sermons are held giving thanks to the Gods and their Bounties, and venerating the connections they are given to kith and kin. Soldiers are honoured in military parades, and they are given extra rations and shorter work shifts as rewards for their commitment to their cause. Rites of Progress are read to students of the faith, and recruits in the armed forces, celebrating how far they have come, mirroring the way the Covenant faith centres around a Journey. 
* The Quadrum of Advocance
** This Quadrum is a celebration of the achievements of oneself, one’s keep, and one’s faith. Progress in constructive, artistic, or contemplative pastimes is celebrated. A time for pride, and a time for competition, as varuous contests are held for art, music, poetry, speaking, duelling, craftsmanship, cooking, etc. A Market is held around the base of the Dreadnaught in High Chairty, and in city centres or outside of Keep gates on planets and colonies. A time for the trading of trinkets, baubles, and other such items of beauty, with food and drink and gifts.
* The Quadrum of Repose
** A time for quiet reflection and recuperation. This Quadrum is marked by solemn contemplation on the deeds taken over the course of the Annum. Selflessness and Honour are the defining traits, and one is expected to be harsh in one’s criticisms, and pious in their repentance of misdeeds. The dead are honoured, and family deeds and the departed are etched onto Keep saga walls. A candlelit vigil is held in important central areas of Keeps and Cities. On High Charity this event would be held around the Keyship, to remember those who have given their lives for the Covenant. This is usually paired with an Annum Sermon from the Hierarchs, detailing what they have accomplished, and what they should strive for in the coming Annum. This is typically the Quadrum in which convicts and traitors are tried and/or executed. Assassinations—somewhat of a Rite of Passage for Sangheili—occur with increased regularity in this Quadrum.