When and how do you know when Eid officially starts?

I'm a college professor and may have an exam that conflicts with Eid. My understanding is the start date depends on the lunar cycle, so isn't known until pretty close to the date. From googling, it looks like it should start on either Friday or Saturday of next week.

- How are you officially notified of the day Eid begins? Who decides?

- When will you know with certainty? A few days before, or the night before?

I always happily waive attendance for students who miss class due to Eid, but I'm a little worried about people trying to take advantage of the situation to avoid taking an exam, so I'm trying to understand the logistics better. Thanks for any info!