Question re: Religious holidays and known dates

[Lsura]

I know in Islam, the actual date of the start of Ramadan isn’t known until a specific sighting of the moon. Are there any other commonly observed holidays in major religions that have a similar situation of “we expect it to start around X date on the western calendar, but can’t say for sure until Y happens”?

Here’s the deal: for staffing reasons I need people to plan ahead for known things where they’ll be out. And I want to make sure people are able to observe their faith as they need to - which means I need to know when I can expect things to generally be known in advance and when I need to have more flexibility.

(I admit that I’m a little crabby this morning about someone’s last minute request for Purim today, which I understand as a known date, but I also could be wrong that it’s generally known in advance. And I want make sure that everyone who reports to me is aware of the need for planning when any known absence is planned for, religious observance or not.)

[Q.Q.Switcheroo]

In Judaism, the Sabbath ends on Saturday evening as soon as three medium-sized stars can be seen in the sky. (If it’s cloudy or you’re in a city with too much light pollution to see stars, educated guesswork is acceptable.)

[Kent_Clark]

I just Googled Purim Dates and got a chart that lists the dates through 2030. Wikipedia has a chart of Ramadan dates that run through 2037. My kitchen calendar doesn’t have Purim on it, but it does have Passover and Ramadan listed, as well as the lunar new year. I don’t think people who want to observe a religious holiday will only find out about it at the last minute.

[kenobi_65]

Disclaimer: I am not Muslim, nor Jewish.

The Wikipedia entry on Ramadan notes this, about its starting date:

Many Muslims insist on the local physical sighting of the moon to mark the beginning of Ramadan, but others use the calculated time of the new moon or the Saudi Arabian declaration to determine the start of the month. Since the new moon is not in the same state at the same time globally, Ramadan’s beginning and ending depend on what lunar sightings are received in each location. As a result, Ramadan dates vary in different countries, but usually by only a day. This is due to the cycles of the moon; the moon may not qualify as a waxing crescent, which delineates the change in months, at sundown in one location but later meet it in another location.[51] Astronomical projections that approximate the start of Ramadan are available.

Because the hilāl, or crescent moon, typically appears one day after the new moon, Muslims can usually estimate the beginning of Ramadan,[54] but many prefer to confirm it by direct visual observation.

So, to me, it sounds like it’s predictable for many years in advance, but it might be give or take a day, depending on local observation. Which may not be helpful to the OP, who seems to need to be able to plan for it ahead of time.