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. This is because it is forbidden to fast on Shabbat, 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 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, 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 Maharil, 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.
Observing ''Seudah Shlishit''
Seudah Shlishit is the third meal of Shabbat, usually eaten on Shabbat afternoon. Traditionally, two loaves of bread 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. Although most chametz has been removed from the household before the search for chametz, enough bread is kept to eat at these three meals. The bread is stored in a location where it does not come into contact with Passover food or dishes. 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. 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. In very rare situations, when another person is in dire need, this chametz 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 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 to leave this extra chametzin 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.
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 percentage of the Eve of Passover on Shabbat occurring 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 three times: in 2001, 2005, and 2008. Future occurrences in the 21st century include 2021, 2025, 2045, 2048, 2052, 2072, 2075, 2079, and 2099.
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, the spring holiday of Shavuot occurs on Monday and Tuesday, the fall holidays of Rosh Hashanah, Sukkot, and Shemini Atzeret-Simchat Torah all occur on Tuesday and Wednesday, and Yom Kippur occurs on Thursday.
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.
