JERUSALEM, April 6 (AP) - In the traditional family meal that marks the beginning of the Passover holiday, the youngest child asks: ``What makes this night different from all other nights?''

This year, there are two extra answers - bomb scares and the calendar.

A tense security situation, including a spate of suicide attacks, has changed pre-holiday shopping habits. And the weeklong holiday begins Saturday night, coinciding with the end of the Jewish Sabbath, sending rabbis scrambling to find dusty tomes that spell out the many rules they must follow, some of them contradictory.

Open-air marketplaces that used to be jammed on the days before Passover, the holiday commemorating the Jews' exodus from slavery in Egypt, were relatively empty. Shopkeepers complained that people were afraid to congregate in large numbers in an outdoor setting, where access for a bomber is easy.

Police reinforcements have been sent to patrol places where people gather on the holiday - marketplaces, malls and vacation spots - but it hasn't eased the concerns.

Benny Levy, 35, who sells soft drinks in a store at Jerusalem's marketplace, said business is down. ``It's very insecure here in the market. It's not a good atmosphere. People here are afraid.''

Instead, Israelis crowded supermarkets, braving the long waits at checkout counters for the reassurance of the security guard posted at the entrance, checking parcels carried by everyone who came to shop.

Shopping is an important part of Passover, not only because of the festive meal that starts the holiday, but also because of the religious requirement to rid one's house of all bread and leavened products. That means buying cleaning products and then more food.

The no-bread requirement leads to a frenzy of spring cleaning. Even many secular Jews who do not follow the strict Orthodox ruling of removing every crumb of bread before the holiday get into the act, shaking out carpets, cleaning furniture, refrigerators and ovens and throwing out food that contains leaven.

Instead of throwing out valuable stocks of food that are not ritually fit for the holiday, Jews have the option of selling the food to a non-Jew. The sale is fictional, but according to Jewish religious law, it must appear to be authentic.

In most cases, Jews go to their local rabbi, fill out a form stating where their food stash is, and make a charitable contribution. The rabbis sell the food to a non-Jew and buy it back after the holiday.

Israelis can also sell their non-Passover food on the Internet. A Web site offers a form to fill in, approved by a rabbi. Sending the form as an e-mail puts in motion the sale of the food.

Many Internet Web sites offer advice to Jews about how to deal with the Sabbath-Passover combination. The Web site of the Har Etzion seminary offers 22 pages on the religious juggling act.

There are strict rules about ridding the house of bread a day before the holiday, but the rules against performing some of the tasks on the Sabbath are just as strict.

Consulting ancient sources, rabbis recommended that observant Jews keep enough bread around, in a special place, to fulfill the commandment of breaking bread on the Sabbath, though the house is supposed to be bread-free by then.

Other recommendations make the Sabbath an extension of Passover. ``It is desirable to cook Sabbath food in Passover dishes, and prepare food that has no hint of leaven,'' Rabbi Dov Weisbart told his congregation in the city of Rehovot.