How the calendar is calculated
Every line you see in the calendar days is computed by the same rules the traditional printed luchos use. This page explains each calculation on its own.
The dates and Yomim Tovim
The Hebrew dates, Yomim Tovim, fast days, Rosh Chodesh and the weekly parsha come from our calendar database.
A day's color shows its kind — a Yom Tov, a minor holiday, a fast day, or a regular day.
The molad
The molad is reckoned with the classic chalakim count: a lunar month lasts 29 days, 12 hours and 793 chalakim. An hour has 1,080 chalakim, so a minute is 18 chalakim.
On Shabbos Mevorchim the molad is shown at the moon sign 🌙 — hover over it and the full molad line appears. The hour is given in daylight-saving time when DST is in effect, the way the printed luchos do.
Night hours are announced on the day the night follows — a molad at 1:54 after midnight of Wednesday is called "Tuesday night".
Shabbos Mevorchim
The Shabbos that falls on the 23rd through 29th of the month is Shabbos Mevorchim — the Shabbos with Rosh Chodesh in the coming week, but never Rosh Chodesh itself.
Tishrei is not blessed, so the last Shabbos of Elul carries no mevorchim line.
Sefiras HaOmer
During sefirah each day shows the count in Hebrew letters — "לג לעומר".
The special Shabbosos
Shabbos Shirah, Chazon and Nachamu follow the parsha itself — Beshalach, Devarim and Va'eschanan.
Shabbos Shuvah is the Shabbos between Rosh Hashanah and Yom Kippur; Shabbos Hagadol the Shabbos before Pesach.
The four parshiyos: Shekalim — the Shabbos on or before Rosh Chodesh Adar (Adar II in a leap year); Zachor — the Shabbos before Purim; Hachodesh — the Shabbos on or before Rosh Chodesh Nisan; Parah — the Shabbos before Hachodesh.
Pirkei Avos
From the Shabbos after Pesach until Rosh Hashanah a chapter of Avos is said every Shabbos. Until Shavuos the six chapters go one per week — that span is always exactly six Shabbosos.
After Shavuos the cycle starts again, and toward the end of summer two chapters are doubled per Shabbos so that chapter 6 lands on the Shabbos before Rosh Hashanah. On Shavuos itself none is said.
BaHaB
After Pesach and after Sukkos the BaHaB fasts are kept — Monday, Thursday and Monday again. The first Shabbos after Rosh Chodesh Iyar (or Cheshvan) is "Mevorchim BaHaB", and the three fasts follow in the weeks after it.
When Rosh Chodesh falls on Shabbos the whole set moves a week later — as in 5786, when BaHaB was announced on the 8th of Iyar.
Yom Kippur Katan
Erev Rosh Chodesh — the 29th of the month — is Yom Kippur Katan. When it falls on Friday or Shabbos it is kept the Thursday before.
Yom Kippur Katan is not kept erev Rosh Hashanah, in Nisan, or in Kislev during Chanukah.
Kiddush levana
"Zayin shleimim" — seven complete days after the molad — is when kiddush levana may begin according to those who wait seven days.
The last time for kiddush levana is half a lunar month after the molad — 14 days, 18 hours and 22 minutes.
The kiddush levana times are shown in standard (winter) clock time all year round, the way the printed luchos do.
The tekufos
The four tekufos — Tishrei, Teves, Nisan and Tamuz — follow Shmuel's reckoning: a year of 365 days and 6 hours, so each tekufah is exactly 91 days and 7½ hours after the one before.
Tekufah times are not adjusted for daylight saving.
Mashiv haruach, tal umatar, v'sein berachah
Mashiv haruach umorid hageshem begins at Musaf of Shmini Atzeres.
V'sein tal umatar begins at night of the 60th day after tekufas Tishrei — usually the evening of December 4th, and December 5th before a civil leap year.
V'sein berachah begins at the first weekday Maariv of Chol Hamoed Pesach — the night after the 16th of Nisan; when that night is Motzaei Shabbos, the line is shown on the Shabbos.
Yizkor
Yizkor is said four times a year: Yom Kippur, Shmini Atzeres, Acharon shel Pesach, and the second day of Shavuos.
Changing the clock
The Shabbos before a clock change carries a reminder — the clock moves forward on the second Sunday of March and back on the first Sunday of November.
Legal holidays
The American legal holidays appear on their days in English, including the "day off" when a holiday falls on a weekend. December 25th appears as "Banks Closed".
Yahrzeits of tzadikim
The major yahrzeits from our yahrzeit directory appear in the calendar days; the full list lives on the Yahrzeits page.
An Adar yahrzeit is observed in Adar II of a leap year, and both Adars fold into the single Adar of a common year.
Erev Pesach, eruv tavshilin and Nittel
On erev Pesach the calendar shows the last time for eating chametz (the end of the 4th hour) and for burning it (the end of the 5th hour), reckoned in Magen Avraham hours — from alos to tzeis — for the city you picked.
Eruv tavshilin appears on erev Yom Tov whenever cooking on Yom Tov for Shabbos will be needed — when Yom Tov falls on Thursday-Friday or on Friday. Minhag Nittel is marked on January 6th.
Candle lighting and Motzaei Shabbos
Friday shows candle lighting (15 minutes before shkiah) and Shabbos shows tzeis hakochavim (72 minutes after shkiah).
How each zman is calculated — see the zmanim page
All of these calculations were checked against a traditional printed luach of 5786. Customs differ — for halachah lemaaseh ask your Rov.