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A perpetual calendar, a calendar designed to find out the day of the week fora given date, employs a rich arithmetical calculation using congruence.Zeller's congruence is a well-known algorithm to calculate the day of the weekfor any Julian or Gregorian calendar date. Another rather infamous perpetualcalendar has been used for nearly four centuries among Javanese people inIndonesia. This Javanese calendar combines the Saka Hindu, lunar Islamic, andwestern Gregorian calendars. In addition to the regular seven-day, lunar month,and lunar year cycles, it also contains five-day pasaran, 35-day wetonan,210-day pawukon, octo-year windu, and 120-year kurup cycles. The Javanesecalendar is used for cultural and spiritual purposes, including a decision totie the knot among couples. In this chapter, we will explore the relationshipbetween mathematics and the culture of Javanese people and how they use theircalendar and the arithmetic aspect of it in their daily lives. We also proposean unprecedented congruence formula to compute the pasaran day. We hope thatthis excursion provides an insightful idea that can be adopted for teaching andlearning of congruence in number theory.

An ethnoarithmetic excursion into the Javanese calendar