Many observances in Jewish law are performed at specific times during the day. The calculation of these halachic times, known as zmanim (Hebrew for "times"), depends on the various astronomical phenomena of the day for the specific locale. Sunrise, sunset, the amount of time between them, and the sun's angular position before rising are all factors that determine the halachic times and "hours" of the day.
[The hour has special meaning in Jewish law. When we say that a certain mitzvah may be performed three hours into the day, this doesn’t mean at three in the morning, or three clock-hours after sunrise. Rather, an hour in halacha means 1/12th of the day. Thus, if the sun rises at 5 am and sets at 7:30 pm, one sha'ah zemanit, or proportional hour, will be 72.5 minutes, and all calculations will use that number.]
Below you will find the times, their meaning and some of their associated mitzvot:
Dawn. Fasts begin at this time. According to Torah law, dawn marks the beginning of the day, and all mitzvot associated with daytime hours -- such as hearing the shofar, taking the Four Species, the recitation of the Shema, or hearing the Megillah -- can now be done. For various reasons, however, the Sages, instituted that the observance of many of these mitzvot should be delayed until Netz Hachamah, or the moment when "one can recognize a familiar acquaintance." According to the Magen Avraham, the calculation of shaot zmaniot begins now.
Earliest time for Tallit and Tefillin:
The halachic description of this time is "when one can recognize a familiar acquaintance" from a distance of approximately six feet. As this is a subjective experience, the time given is approximated to an 11 degree depression of the sun. This is also the earliest time one can say the Morning Shema.
Sunrise. The calculation of shaot zmaniot begins now according to many opinions (including the Alter Rebbe in his Siddur), and all the Halachic times provided by Chabad.org reflect this view. Those who wish to pray vatikin, start the recitation of the amidah at this time.
Three shaot zmaniot into the day. Latest time of the day to fulfill the biblical requirement to recite the Morning Shema. B'dieved (if one missed this time), one should still recite Shema with its blessings until chatzot.
Four shaot zmaniot into the day, ideally the latest time for shacharit, the morning prayer. However, if this time was missed, shacharit may be recited until chatzot.
Midday; the halfway point between sunrise and sunset. Half-day fasts end at this time.
Half a shaah zmanit after chatzot. This is the earliest time one may recite minchah, the afternoon prayer.
Nine and a half shaot zmaniot hours after sunrise. According to certain halachic authorities, it is preferable to wait until this time before praying minchah.
One and a quarter shaot zmaniyot before sunset. According to Rabbi Judah this is when halachic nighttime begins. Therefore, if one chooses to follow his opinion, one recites minchah before plag haminchah and then maariv (the evening prayer) may be recited anytime after the "Plag". This is also the earliest one may bring in the Shabbat on Friday afternoon.
Candle Lighting time:
The accepted custom is to light Shabbat and Yom Tov candles 18 minutes before shkiah (sunset). Some communities have adopted earlier times as their unique custom for candle lighting time.
Sunset. The latest time for minchah, the afternoon prayer, and all mitzvot associated with daytime hours. B'dieved (if one missed this time) one may still recite minchah, and do all "daytime mitzvot" until Tzeit Hakochavim (although the blessing on the mitzvah would be omitted if done after Shkiah).
The Jewish 24 hour day begins at nightfall. However, the technical definition of nightfall is unclear. It can be as early as Shkiah, or as late as Tzeit Hakochavim. Therefore, the time following shkiah and before tzeit hakochavim is called bein hashmashot. Many laws relate to this period and it can be categorized as either the previous or the next day.
The time when three stars are visible in the sky and nightfall is complete. Earliest time for maariv (evening service) according to Rabbi Judah's rabbinic counterparts. Earliest time for reciting the evening Shema and Counting the Omer. A woman who has completed her cycle of seven pure days goes to the mikvah after this time.
There are differing opinions when Tzeit Hakochavim takes place. Out of consideration for people's comfort, and considering that the fast days are rabbinic decree, not Torah law, we rely on a slightly earlier opinion concerning the end of day fasts (aside for Yom Kippur).
Shabbat End time:
Shabbat and festivals end, and "weekday" work may resume, at this time. A stricter calculation of Tzeit Hakochavim is used. Known as the appearance of "three small stars," it coincides with the sun’s descent to 8.5 degrees below the horizon. This stringency also ensures that we do not accidentally violate the sanctity of the day, and that we fulfill the obligation to add time from weekday onto the Shabbat or holiday.
Proportional hour, i.e. an hour according to halachah. Total daylight hours divided by 12.