Nedarim, Chapter 8, Mishnah 3. In the previous Mishnah we learned that one who says "until" a certain time means until that time arrives, at the beginning of that period: one who says "until Pesach" is bound until Pesach begins. But one who uses the wording "ad sheyehei" - until it will be - means until Pesach ends.
The novel point of our Mishnah:
Our Mishnah clarifies that even the wording "ad sheyehei" is sometimes understood as referring to the starting point alone, namely in a case where it is not known how long that period lasts. Pesach has a fixed and known duration, and therefore when a person says "ad sheyehei" it is understood that he means until it ends. But when it is unclear how long the period will last, we cannot assume that the one who took the vow intended to forbid the item upon himself for that entire span of time, since he does not know when it will end.
The wording of the Mishnah:
"ad haKatzir" - until the harvest, meaning the harvesting of the grain.
"ad haBatzir" - until the vintage, the picking of the grapes.
"ad haMasak" - the term used for the gathering of the olives.
With all of these expressions - "eino asur ela ad sheyagia" - he is bound only until it arrives, meaning only until that season arrives and begins.
"Zeh haklal" - this is the rule:
"kol shezmano kavua" - anything whose duration is fixed and known: "amar ad sheyagia - asur ad sheyagia" - if he said "until it arrives," he is bound until it arrives, whereas "amar ad sheyehei - asur ad sheyehei" - if he said "until it will be," he is bound until it is over.
"v'chol she'ein zmano kavua" - anything whose ending point is unclear, including the seasons of the grain harvest, the vintage, the olive gathering and the like: "bein amar ad sheyehei, bein amar ad sheyagia - eino asur ela ad sheyagia" - whether he said "until it will be" or "until it arrives," he is bound only until it arrives, that is, until that season arrives and begins.
The reason for this: from that point onward it is not known how long the period will last, and therefore we cannot say that the one who took the vow intended to include that entire span of time. His intention was to the starting point alone.
In summary: in this Mishnah we learned that the distinction between "ad sheyagia" and "ad sheyehei" applies specifically to something whose duration is fixed; but with something whose duration is not fixed, the two expressions are equivalent, and the one who took the vow is bound only until the arrival of that season.