Beitzah 19A

ואיבעית אימא רבנן, וכולה מתניתין בשבת.

And if you wish, say instead that the Mishnah is in accordance with the opinion of the Rabbis, and on a Festival one is permitted even to immerse an impure vessel on account of its water in order to purify the vessel. And the entire Mishnah is referring to Shabbat, when it is prohibited to immerse an impure vessel, but it is permitted to purify impure water by bringing it into contact with pure water in a stone vessel.

תנו רבנן: כלי שנטמא מערב יום טוב ⁠— אין מטבילין אותו בין השמשות.

§ A baraita states: With regard to a vessel that became ritually impure on the eve of a Festival, one may not immerse it during twilight, a time period of doubtful status whether it is a weekday or a Festival, due to the possibility of violating the prohibition against immersing vessels on a Festival.

רבי שמעון שזורי אומר: אף בחול אין מטבילין אותו, מפני שצריך הערב שמש.

R' Shimon Shezuri says: Even on a weekday one may not immerse an impure vessel during twilight because the vessel requires sunset. After an impure vessel is immersed, it remains ritually impure for certain purposes until the sun has set and the stars have come out. If one immerses an impure vessel during the twilight period, then owing to the uncertainty as to whether it is day or night, he must wait another full day, until the next sunset, before using the vessel. It is therefore preferable not to put oneself in a situation where one might come to use a vessel before its purification process has been completed.

ותנא קמא לא בעי הערב שמש? אמר רבא: אשכחתינהו לרבנן דבי רב דיתבי וקא אמרי, במחשבתו נכרת מתוך מעשיו קמפלגי. והיכי דמי ⁠— כגון דנקיט מנא בידיה, ורהיט ואזיל בין השמשות לאטבוליה.

The Talmud asks: And does the first tanna not require sunset? It is clear that this is required. Rava said: I found the rabbis of the school of Rav sitting and saying with regard to this issue that they disagree about whether or not to accept the principle that one’s intention is evident from his actions. And what are the circumstances of this dispute? They are, For example: a case where one was holding a vessel in his hand and running along at the time of twilight to immerse it.