In my on-going efforts to help you greet (and not offend) your Jewish friends and colleagues during Jewish holidays, let's talk Passover!
Passover starts at Sundown on Saturday (first full moon in April) and goes for seven or eight days, depending on who you talk to. It is an observance of the story of the Jews leaving Slavery in Egypt. They were in such a hurry they couldn't wait for their bread to rise, so they had to eat crackers. It is roughly analogous to the American holiday of Thanksgiving in that it doesn't have the profound significance of Rosh Hashanah or Yom Kippur (which are like Christmas and Easter), but people still want time off from work to prepare and be with family.
If you've seen "The Ten Commandments" with Heston, you know the story of Passover.
Ecumenically, it concerns freedom from boundaries. The eating of unleavened bread confers the urgency that the forces of liberation sometimes bear down upon us.
Some very orthodox Jews don't work for the first and last day, some specific groups have other specific guidelines for behavior. Almost all Jews observe it with a ritualized meal called a Seder, which is cool because you down two glasses of wine before you get any food at all, and four throughout the meal. There are readings, the story is told, specific foods are eaten at different points to confer meaning to the readings. It's a hoot, if you ever get invited to one, go.
The Hebrew greeting is "Chag Sameach," which I remember, ironically as "hog sammich." Don't embarrass yourself, say "Happy Passover!" A nice gift is a floral table centerpiece.