(CNN) On Palm Sunday, Christians throughout the world celebrate the day Jesus triumphantly entered Jerusalem days before his crucifixion.

Crowds left palm fronds and clothing in his path as a sign of praise and respect.

But millions of American Christians will observe Palm Sunday at home this year , as the coronavirus pandemic has forced most US churches to hold services online because of stay-at-home orders.

Here's what you need to know about the Sunday that launches the holiest days for the world's Christians:

When is it celebrated?