Beitza 19 a, b

The concept of twilight is fuzzy.  Twilight does not have a measurable, exact time or status.  Unlike sunset, where the sun disappears beneath the horizon, or evening, where we can visually locate stars, twilight is purely subjective: I can't define it, but I know it when I see it.  Twilight might best be identified after the fact; once the sun has set, we know with certainty when twilight ended.  But in any given circumstance, can we define when twilight begins?

If we err and we immerse a vessel after twilight on the eve of a Festival, what is the consequence?  If we are warned, does the consequence change?  And if we knew what we were doing, how might that change the consequence?

The rabbis discuss whether or not we can assume a person's intentions based on his or her actions. If someone is running to immerse a vessel at twilight, for example, should we assume that s/he knows about the laws regarding immersion at twilight before a Festival?  Or, perhaps that person is running because s/he needs to know whether a creeping animal the size of a lentil bulk can impart ritual impurity.  

A new Mishna brings us back to offerings on Festivals.  Beit Shammai say that we can bring peace-offerings but not put our hands on their heads during a Festival.  Putting our hands on an animal is similar to using an animal for a separate purpose, which is not allowed on Festivals.  They also say that burnt offerings are prohibited on Festivals, as burnt-offerings are meant to be consumed by people and on Festivals, burnt-offerings are eaten on the altar only.  Guess what Beit Hillel say?  Both peace-offerings and burnt-offerings are permitted on Festivals.

The rabbis argue about what Shammai and Hillel are actually debating.  Is this about a larger principle, like the meaning of a peace-offering?  Or is this in fact an argument about the requirements on a Festival?

Steinsaltz teaches in a note that a thanks-offering follows a miraculous event.  It is comprised of 10 loaves of leavened bread and 30 loaves of three different types of matzah.  It is to be consumed that day or the following day.  Thus thanks-offerings are welcome on Sukkot but not on Pesach or Shavuot, which restrict bread and/or the timing of consumption.  In addition, the rabbis argue, Sukkot is a time to rejoice, which we do with thanks offerings.  They suggest a number of other arguments to justify this understanding.

We learn about the halacha of not delaying, denai baal ta'acher.  When we vow or promise to do something and it is not done by the following Festival, we have negated a positive mitzvah.  But if we have not kept our promise after three Festivals have passed, we have transgressed the negative mitzvah of "you will not delay".  The rabbis imagine situations where we could invoke an exception.  For example, if we promise to do something between two Festivals but we cannot so so, we might be excused - depending on the circumstances.