How early may one daven Shacharis?

Tefilas Shacharis was instituted as a corresponding service to the Korbon Tamid Shel Shachar--the morning Tamid. The shechitah of the Tamid took place at alos hashachar (daybreak). It follows that one may daven Shacharis at that point as well. Chazal, however, instituted that the appropriate time to daven is at hanetz hachamah (sunrise). Under pressing circumstances, one may begin davening at alos. It should be noted that although Shemoneh Esrei may be recited at that point, Kriyas Shema may only be said once it is light enough to recognize a casual acquaintance (misheyakir). If one would like to daven early, he may begin Pesukei D’zimrah before misheyakir, put on tallis and tefillin at misheyakir and then begin Birchos Kriyas Shema and continue through the end of davening.

סימן פ"ט ס"ק א' וס"ק ד'

What time is davening on Shavuos morning?

An example of pressing circumstances in which a person can daven before hanetz is the case of someone who must get up early to travel with a group that will not wait for him to daven at the proper time. Another example mentioned in the poskim refers to Shavuos morning. If the tzibur stayed up all night and people are so tired that it would be hard to wait for hanetz in shul, they may daven early. Although they could technically go home to sleep and come back later to daven, reconvening the tzibur would be difficult.

ס"ק א' וע"ע בס"ח

How to calculate zman tefilah?

The zman for Shacharis extends until four halachic hours of the day have passed. A halachic hour is calculated by dividing the daytime hours into twelve equal parts. There are two opinions on how to determine daytime hours. Some say that the day extends from alos hashachar until tzeis hakochavim, while others say that day begins at hanetz and ends at shkiyah. The time for the completion of Shacharis depends on which of these opinions one follows. Shemoneh Esrei must be completed by this time. Chazoras Hashatz must be completed by zman tefilah as well.

ס"א ס"ק ה' ביאורים ומוספים דרשו 6,8,9

• Generally, when a person immerses in a mikvah there cannot be any chatzizah (blockage) between the body and the water. A chatzizah which a person does not mind having on the body is only forbidden if it covers most of the body. If a person is particular about the chatzizah, then it is an issue even if it is only on a small part of the body. When immersing for keri, however, one may have a chatzizah if it does not cover most of his body.

• There are some women who are careful not to visit a cemetery when they are pregnant.

• According to some Rishonim, a woman who is in a state of impurity should not walk into a shul. The accepted custom does not follow this ruling. Nonetheless, a woman in this state should not gaze at the words of the Sefer Torah when it is lifted for Hagbah.

• What is the deadline for davening Shacharis?

• May one daven after midday before the time for Mincha?

• Are greetings permitted before Shacharis?