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Every culture has rhythms, such as holidays, that have nothing to do with the Gregorian calendar, yet most business cycles are based on the latter. Religious holiday vacations tend to pop up at seemingly random dates, but in fact often follow a lunar calendar: Ramadan, Passover, Easter, and Hanukkah are typical examples. We as professionals tend to believe that time is uniform, that a day is a day and a week is a week, yet Project Managers are taught to pretend that December as a month has only 10 days (of effective work). In the meantime Scrum Sprints follow fixed, integral numbers of units of time that derive from the Gregorian calendar and are confounded by the reality of local holidays. And “local” differs from culture to culture and country to country. Denmark shuts down for the month of July; Germany tends to split its summer siesta across two months. Australia’s holiday season is different yet again, and heaven help the poor enterprises that believe in multisite development and are distributed between Europe and Australia. We won’t even mention India, whose holidays follow the sun and the moon.
While we acknowledge the rhythm of the Sun in the Daily Scrum, we often ignore larger rhythms of life like the phases of the Moon and the local seasons. Human beings are not machines who can deliver the same every hour and day all time of year, just as European Project Managers deeply discount December. So we work in day time and sleep at night. We have weekends, holidays, and vacations. These create annoying irregularities for Sprint Planning, Roadmaps, and the concept of velocity.
Early definitions of Scrum recommended that Sprints run on about monthly cycles; more specifically, on 4-week cycles. In most cultures a Scrum “month” is not a calendar month. In monochromic cultures  a day can be said to be 86,400 seconds, and an “ideal month” is a collection of 30 such days. But these “months” are not the calendar months that vary in length: Sprints are always the same length. Scrum scheduling more closely follows the cultural rituals of the Scrum framework than it does any business calendar. (If you don’t believe this, quickly answer how many business months you have in a year and then how many Sprints you have in a year, hopefully without having to do any math.) So cadences in Scrum can be defined in terms of ceremonies, rituals and benchmarks from the framework rather than from the calendar. And the recommended Sprint length from Scrum’s early days was closer to the periodicity of our largest astral body than to the rhythms of the Gregorian calendar. (Today, two-week rather than four-week Sprints are the norm.)
Working outside natural rhythms upsets human subjects so badly as to suggest it has been the cause of numerous disasters such as Chernobyl, Three Mile Island, and the Exxon Valdez. 
Define Sprints with beginning and ending boundaries falling on the New Moon and Full Moon. This works perfectly for two-week and four-week Sprints. Use common sense when accommodating non-lunar local events such as Christmas, Mid-summer, Thanksgiving, or other work pauses that are customary to the local culture. It can be a nice picture of incremental work.
Get the team involved in putting together a long-term Sprint calendar that accommodates holidays and traditional vacation times. For example, many European subcultures take vacation together during the same month or two weeks.
Those cultures that follow the Hirji calendar (التقويم الهجري) or other lunar calendars are already halfway there. Many of the religious holidays that otherwise upset the normal Gregorian cadence also follow a lunar calendar, and the chances that they align well with Sprint boundaries is much higher if you Follow the Moon.
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This scheduling establishes a regular cadence, and universally sets expectations about when the next Sprint will end. Regular Product Increments will be synchronized with broadly understood cultural rhythms. Team members’ energy cycles will likely conflict less with naturally occurring cycles in the culture. This helps the team advance some of the deeper human nuances of Greatest Value.
This is a HA pattern. For SHU use Fixed-Date PBI. In RI we can playfully conjecture that you are enough harmonized with your environment that you might follow the moon naturally.
Sally Helgesen  writes about the importance of natural cycles to well-being:
The sociologist Eviatar Zerubavel, in a fascinating study entitled Hidden Rhythms, observes that humans experience time in two entirely different ways: as the cyclical rhythms of nature, governed by recurrent seasons; and the repetitive rhythms of the machine, regulated by schedules, calendars, and clocks. He notes that being able to perceive time as cyclical is essential to our well-being, a crucial manifestation of our connection with nature in which time always moves in cycles.
See also Sprint.
 Edward T. Hall. The Dance of Life. New York: Bantam Doubleday Dell Publishing Group, re-issue of 1988.
 Steven H. Strogatz. Sync. New York: Hachette Books, 2014, p. 85.
 Sally Helgesen. Everyday Revolutionaries: Working Women and the Transformation of American Life. New York: Doubleday, 1998, p. 140.
Picture from: Spirit-Fire, www.flickr.com/photos/spirit-fire/6801036620, (under CC BY 2.0 license).