  A perpetual calendar, a calendar designed to find out the day of the week for
a given date, employs a rich arithmetical calculation using congruence.
Zeller's congruence is a well-known algorithm to calculate the day of the week
for any Julian or Gregorian calendar date. Another rather infamous perpetual
calendar has been used for nearly four centuries among Javanese people in
Indonesia. This Javanese calendar combines the Saka Hindu, lunar Islamic, and
western 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 Javanese
calendar is used for cultural and spiritual purposes, including a decision to
tie the knot among couples. In this chapter, we will explore the relationship
between mathematics and the culture of Javanese people and how they use their
calendar and the arithmetic aspect of it in their daily lives. We also propose
an unprecedented congruence formula to compute the pasaran day. We hope that
this excursion provides an insightful idea that can be adopted for teaching and
learning of congruence in number theory.
