For the source text click/tap here: Nedarim 61
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A new Mishna tells of a vow that forbids something to a person until the grain harvest, or until the grape harvest, or until the olive harvest, the vow is in effect until that date arrives. The principle is that a time-fixed event ends a vow upon its arrival. However, if a person vows that s/he is forbidden from something "until it will be [the grain or the grape or the olive harvest]", that vow is valid until the event ends. And if an event has no fixed time associated with it, then the vow is in effect only until the occasion arrives.
The Mishna goes on to say that if a person says "until the summer (kayitz)", or "until it will be summer" regarding the end of his/her vow, that vow is honoured until the people begin to bring fruit into their homes in baskets. If s/he says, "Until the summer has passed", however, the vow stays in effect until people put away the knives that cut harvested figs. Thus a custom helps the rabbis to determine when the summer begins and ends; when a vow begins and ends.
We explore the connection between Shavuot and harvest festivals as well as the inner metaphor of harvesting in spiritual traditions.