Today, parts of the World observe the New Year of the Islamic Calendar. The Islamic Calendar is a lunar calendar, i.e. it complies with the phases of the moon and has a total of 354 or 355 days per (lunar) year. Many Muslim holidays are calculated on basis of this calendar, which creates a drift relative to the Gregorian date: Each year, each Muslim holiday starts about 11 days earlier in the Gregorian calendar than the year before, and Christmas on the other hand happens about 11 days later, calculated in the Islamic calendar.
Secular reasons (like the calculation of agricultural terms) necessitate the use of the Gregorian calendar in parallel. The parallel handling of two (or even more) calendars is no mean feat. We want to appreciate it with today's level, “Maze For Two” by Ronald Lamprecht, and wish a Happy New Year to all those observing 1 Muharram!