Once every 27 or so years, major holidays of several faith traditions meet up for a reunion scheduled by the heavens.

The get-together comes by way of religious calendars alternately determined by the sun, moon, and sometimes, both.

Next month, Ramadan, Passover, and Easter, along with celebrations by Buddhists, Jain, Sikh and Hindu families will fall within weeks, sometimes days, and in some cases, on the same day, in April.

“It happens [several] years in a row, about every 30 years, '' said Dr. Benjamin Dreyfus, an associate professor of physics and astronomy at George Mason University in Fairfax, Va., and administrator of  the Hebrew Calendar Facts Facebook page. The holidays are currently synched in one of those cycles, Dreyfus continued, but by 2024, Ramadan will have moved on.

Most of the holidays are relatively stationery, intentionally anchored in the spring, a season of renewal and new beginnings. But Ramadan, the holiest month in the Islamic tradition, is roughly 11 or so days earlier every year.

Islam uses a lunar calendar which is based on the phases of the moon. A year is 354 days. A solar calendar, such as the Gregorian one followed by most of the world, is based on the time it takes for the earth to orbit around the sun. A year is 365 days. Because the lunar year is shorter, each year Ramadan falls earlier in the Gregorian calendar.

At sundown on Friday, Muslims around the world are expected to begin celebrating Ramadan, which commemorates the revelation of the Quran, the Islamic holy book, to the Prophet Muhammad.

Muslim families will fast during the day and read the Quran. The month-long observance ends with the Eid-al-Fitr, which means festival of breaking fast. The celebration includes community gatherings, exchanging gifts, barbecues and dining out, but imams and leaders in other faith traditions celebrating holidays are proceeding cautiously as the pandemic appears to be waning.

This year Ramadan starts two weeks before Passover and Easter, holidays anchored in springtime.

Passover, the Jewish festival that celebrates the Israelites’ Exodus from Egypt and freedom from slavery, begins at sundown on Friday, April 15, and is observed for seven or eight days depending on the denomination and location. Families observe the holiday by gathering for seders, ritual meals that include symbolic foods which represent the Passover story.

The Jewish calendar is both solar and lunar. Strict adherence to a lunar year means that holidays would shift and eventually fall in different seasons.

But “the ancient rabbis didn’t want to lose the connection of holidays to particular seasons,” said Rabbi Adam Zeff, of Germantown Jewish Centre in Mt. Airy. “Passover is associated with redemption and new life, so they wanted to keep it in the spring,”

So, the rabbis inserted a “correction,” Zeff said. A month is added to the Jewish calendar, before Passover, during leap years. The adjustment ensures that Passover always comes in springtime.

This year the holiday starts during the Christian Holy Week before Easter when Christians commemorate events such as the Last Supper, the last meal Jesus Christ shared with the apostles before his death on Good Friday. This year, the beginning of Passover and Good Friday fall on the same day. Some Christians will observe Good Friday by attending services that highlight the seven last words Christ is said to have uttered from the cross.

Days later, on Sunday, this year on April 17, Christians observe Easter when they believe Christ was resurrected from the dead. Followers attend church service, sometimes at sunrise, along with family dinners, Easter egg hunts for children.

The First Council of Nicaea, a meeting of Christian bishops in 325 AD, took the first steps to standardize Easter’s place on the calendar, which is now after the first day of spring, on the first Sunday immediately following the first full moon. In Eastern Orthodox churches, Easter, this year on April 24, is usually at least a week after the date observed by Western Protestants and Catholics. Eastern churches used a different calendar to calculate Easter.

Also clustered around the Easter/Passover weekend are important days for Buddhist, Hindu, Sikh and Jain believers. Vaisakhi, which celebrates the solar new year for Hindus and Sikhs, falls on April 14. Mahavir Jayanti, also on April 14, celebrates the birth of Lord Mahavira, believed to be the last great sage of the Jain tradition. On April 16, devotees of Theravada Buddhism will observe their new year. Its principles are practiced mostly by Buddhist families with Thai, Cambodian, Laotian and Myanmar roots.

Some families’ observance of the holiday by washing the feet of the elders has evolved into joyous celebrations with water that include splashing friends  and engaging in water gun fights, said C. Pierce Salguero, a professor at Penn State’s Abington campus and author of “Buddhish: A Guide to the 20 Most Important Buddhist Ideas for the Curious and Skeptical.”

 “I have driven through a city on a bike and just been doused by revelers in the streets soaking everybody,” Salguero said.