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There are a few areas where we use the rule מקצת היום ככולו - "part of the day is considered as if it were the entire day"

I have some questions regarding how this rule works:

  • Is there a general rule, situation or scenario as to when this rule can be applied? For example, we don't say, "observing part of Yom Kippur is as if you observed all of it. Therefore, you can eat after noon time."

  • The only place, offhand, that I can think of where this rule is applied is regarding the observance of shiva. On the last (7th) day, mourners don't complete the entire 7th day, but are done in the morning. My question is - if the day has already begun the night before (at sundown), why can't the last day of shiva end an hour (or whatever period) after nightfall?

  • Other places we apply it are Nazir and Zav/Zavah. Seemingly the common denominator is counting days. – Double AA Nov 21 '16 at 17:18
  • Some rishonim think Shiva does end just after nightfall – Double AA Nov 21 '16 at 17:19
  • Interesting to note that there are those who have the custom to fast 26 hours on Yom Kippur. And say that part of the 26th hour is like the whole 26th hour – Menachem Nov 22 '16 at 6:46
  • (Another place it comes up is the mourning-like customs of the s'firas haomer period.) You may wish to separate your two questions into two posts: they're quite disparate IMO. – msh210 Nov 22 '16 at 13:34
  • Some refrain from milk for more than five hours after eating meat. I've heard (not from any reliable source) that this is because they wait six hours but part of the sixth is like the sixth. (cc @Menachem because you also cited an hour-based custom.) – msh210 Nov 22 '16 at 13:35
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You touch a Suggia, which is addressed mainly in masechet Nazir, Pesachim, Megila and Nidda.

The nafka mina is mainly when can we complete a count of days. For zav and zava, metsora, death uncleanness and mila in Tora, and for nezirut for more dinim which are rabbinical as mourning H' ishmor.

The question of Miktsat Hayom Kekulo occurs when a day can end the status of some din and start immedialely further a status for an other din contradictory with the first. There is a concurency between two counting and we say that this day is counted as a day for each dinim, since each of those dinim have a part of the day for him.

Concerning Shiv'a, it seems that even Man Deamar Min Hatora that there is no Miktsat Hayom Kekulo may aggree for shiv'a which is derebaban.

In Miktsat Hayom kekulo there are several degrees. The maximum is Rabbi Yosse who count a part of the night and doesn't require at least a part of the day. You address this question. The suggia is in Nazir 15b-16a, Pesachim 81a, concerning sfirot of zav and zava.

In your examples, Yom Kippur, and why not, Shabbat, there is no concurrency between 2 dinim. I think that the problem is if more than "1" is already in the category of "2". This question begin only when we have a count of more than one day. May be that if you reached the second day, you are already good enough. Gemara clearly doesn't ask for one day only. Few than one day is counted in hours or in Onot which are half days. More than one day is counted with "day" as unity.

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