Eve of Passover on Shabbat
In Judaism, when the Eve of Passover (Hebrew: ערב פסח, "Erev Pesach") falls on Shabbat, special laws regarding the preparation for Passover are observed. Fast of the Firstborn. When the Eve of Passover falls on Shabbat, the Fast of the Firstborn customarily takes place on the preceding Thursday, instead of the day before (Friday). This is because it is forbidden to fast on Shabbat (except when it coincides with Yom Kippur), and it is preferable not to fast on Friday. There is some debate over whether a "siyum" on such a Thursday would be enough  to remove the requirement of fasting, or if it would only move the requirement to fast to Friday. Search for "chametz". Normally, the search for "Chametz" (leavened bread) occurs on the night of the 14th of Nisan, which is one night before the start of Passover. When this night is a Friday, the search for "chametz" takes place one night earlier (on the 13th), since use of a candle and the act of burning "chametz" are forbidden on Shabbat. The "chametz" is then burned on Friday morning. It is normally forbidden to eat before conducting the search for "chametz", which  is carried out by the firstborn. This firstborn may be fatigued or uncomfortable due to having just observed the Fast of the Firstborn. According to the "Mateh Moshe" and "Mahari"l", a firstborn who is fatigued or uncomfortable from the fast may eat some food before the search, or another person may be appointed to perform the search on behalf of the firstborn. After completing the search, we recite the first kol chamira, a text found in any haggadah or Passover prayer book. We recite this passage to nullify any chametz that may not have been found. Chametz to be eaten  on Friday and Shabbos is stored in a safe place, where it will not get mixed with the Pesach foods. The Shabbos Meals. The first of the three meals is consumed on Friday evening, as usual. On Saturday morning, morning services at synagogue are held earlier than usual. Following services, a second meal is held and finished quickly. After a pause, a third meal is begun, in which the remainder of the "chametz" is consumed. You should not eat the challah over the Pesach utensils and tablecloths. Prepare the exact amount you need for the members of your household, for  you do not want to be stuck with leftovers. It is best for you to serve small rolls, as they leave fewer crumbs, and to eat over a tissue or napkin, which can be flushed down the toilet when done. Wash your hands from any chametz residue, and then continue with your Shabbos meal. For those who are concerned about having chametz around, there is also the option of eating kosher for Passover egg matzos (which may be preferable for those with braces). Consult your rabbi to know how much of it must be consumed. You may not eat any  chametz after the fourth halachic hour on Shabbos morning. Consult a Jewish calendar for the exact time in your area. One may not eat Passover matzos this entire day. We eat the meal early in the morning, so that the challah is finished at the proper time. We use the same procedure Shabbos day as we did Friday night. After you finish eating the challah, wash your hands and rinse your mouth well, making sure to leave no residue of chametz. Use a dry, chametz toothbrush, and remember that no toothpaste may be used. You must get rid of any  leftover chametz by crumbling it over the toilet and flushing it. When done, recite the second kol chamira. You then continue the Shabbos meal, eating only Passover foods and no matzah. Many people utilize the long Shabbos afternoon as an opportunity to familiarize themselves with the text and translation of the Haggadah in advance of the Seder. It is also a good idea to take a nap, but be sure not to verbally express that you are taking a nap in preparation of the Seder. Observing "Seudah Shlishit". "Seudah Shlishit" is the third meal of Shabbat, usually eaten on Shabbat  afternoon. Traditionally, two loaves of bread ("challah") are served with the three Shabbat meals, including "Seudah Shlishit". However, when the Eve of Passover falls on Shabbat, the restriction on consuming such "chametz" begins on Saturday morning; to include bread with each of the meals, all three must be consumed before the restriction comes into effect. For seudah shlishis (the third meal), one should eat meat, fish or fruit sometime in the afternoon. Do not eat too much; you want to have an appetite for the matzah at the Seder. Any "chametz" that remains following the completion of three meals is  disposed of. If "chametz" is found after Passover begins, it is customary to bag it and place it outdoors until it can be disposed of in proper waste receptacles following the third day of Passover. (It is not disposed of immediately because of the restriction of working or carrying money on the first and last two days of Passover, similar to the restrictions on laborious activities on Shabbat.) In very rare situations, when another person is in dire need, this "chametz" may be given away instead of thrown out. It is advised that each community assess the overlooked "chametz", and , if the amount is substantial, donate double in fresh "chametz" (bread and bread products) directly to those in need, through organizations that feed those in need, immediately following the second day of the Passover. The actual extra "chametz" would likely become too stale for human consumption by the time it could reach those in need, so it is considered a "mitzvah" (good deed) to leave this extra "chametz" in public areas as food for birds and squirrels. Some national laws do not allow this because it may attract dangerous or unwanted wildlife, so instead the extra "chametz" should simply be  thrown out in outdoor waste receptacles. This practice is decided by the rabbis in each community. Individuals generally consult with a local rabbi in the event of finding any substantial "chametz" during Passover. Do not make any preparations for the Seder, such as setting the table, washing the dishes, or making the salt water, until after Shabbos is over. Insert Havdalah into the Kiddush at the Seder. Follow your Haggadah for the correct text and procedure. Frequency. While the coincidence of the Eve of Passover and Shabbat can occur as often as three times in a decade, it is also  possible for as many as 20 years to pass between two instances. The likelihood of the Eve of Passover occurring on Shabbat on any given year is 11.5%. During the 20th century, the Eve of Passover fell on Shabbat 12 times: in 1903, 1910, 1923, 1927, 1930, 1947, 1950, 1954, 1974, 1977, 1981, and 1994. In the 21st century, it has occurred four times: in 2001, 2005, 2008, and 2021. Future occurrences in the 21st century include 2025, 2045, 2048, 2052, 2072, 2075, 2079, and 2099. Other holidays. Other Jewish holidays in the same year. For years in which the  Eve of Passover falls on Shabbat, some other Jewish holidays are also observed irregularly. Purim, which comes earlier in the year, occurs on Friday (beginning Thursday night, and making Purim a three–day holiday in Jerusalem), the spring holiday of Shavuot occurs on Monday and Tuesday (beginning Sunday night), the fall holidays of Rosh Hashanah, Sukkot, and Shemini Atzeret-Simchat Torah all occur on Tuesday and Wednesday (beginning Monday night), and Yom Kippur occurs on Thursday (beginning Wednesday night). Other Jewish holiday alteration. In years when the Eve of Passover is on Friday and the first day of Passover is  on Shabbat, which happens more commonly than the Eve of Passover falling on Shabbat itself, the 17th day of Tammuz and the ninth day of Av will fall on a Saturday; since fasts other than Yom Kippur are not observed on Shabbat, the fasts of the Seventeenth of Tammuz and Tisha B'Av will be postponed to the next day (Sunday - the eighteenth of Tammuz and the tenth of Av, respectively).