On fast days that begin in the morning, (Asara B'Teves, Taanis Esther, Shiva Asar B'Tamuz, Tzom Gedalya) if you intend to eat when you awake, until what time may you eat in the morning? Does it depend on Shaos Zemanios?
2To be clear, dawn is the latest time these fast days can start. They usually start when one finishes eating during the preceding night. If one goes to sleep with intending on sleeping through until morning and then wakes up by accident, most authorities forbid him from eating even before dawn because the fast already started for him. See Shulchan Aruch 564– Double AA ♦Jan 4, 2012 at 15:48
myzmanim.com writes: "Eating of a settled character - אכילת קבע - may not be started during the half hour immediately preceding dawn. Please consult your Rabbi for details."– CuriouserJan 4, 2012 at 18:29
@Curiouser That is a rule regarding Kriat Shema. You can't start a seudah within half an hour of the time of chiyuv. Presumably this restriction would not apply to women who are pturot from kriat shema.– Double AA ♦Jan 4, 2012 at 20:21
Re the first response, the halacha as described in Shulchan Aruch 564 is quite different than presented. First, fasts always start at amud hashachar (72 minutes l'chumra, 90 minutes super l'chumra). The only exception is where one fell into a deep sleep with no intention of eating afterward. However, most people either set their clock to davka wake up early, or else hope that they'll wake up early enough to have a bite before the fast starts. In that very common case (possibly 100% nowadays), everyone agrees one may eat. Moreover, the Rema there on 564 says that we always have an intention to drink during the night, so there's no need to make a t'nai (condition) in order to drink if one wakes up unexpectedly early. So even for the machmirim, one can drink right before amud hashachar.
2If 72 minutes is l'chumra, then what is ikar hadin?– Double AA ♦Dec 23, 2012 at 8:37
Minor fast days start at dawn (Alote HaShacar) which is about 72 minutes before the sun rises.
According the Magen Avraham Alot Hashacar is the pre-sunrise event that occurs when the rays of the sun first become evident.
The night ends at "Alot haShachar", the "rising of the morning". It has two halachic definitions: Most rule it is 72 minutes before dawn, some use the solar equivalent--16.1o (degrees) below the horizon. The latter would come dawn+72 min if the sun were up for exactly 12 hours that day. In the summer it would be longer, in the winter, later. Others use 90 solar minutes.(I'm not sure about this source)