Nazir chapter 3, mishnah 6. The mishnah deals with someone who accepted upon himself a long nezirus - more than thirty days - while he was outside the Land of Israel: "Mi shenazar nezirus harbeh vehishlim es nezirusso ve'achar kach ba la'aretz" - that is, he completed the nezirus he had accepted upon himself outside the Land of Israel, and afterward came up to the Land.
The background of the law - the tumah of lands outside Israel:
A decree of tumah was issued upon lands outside Israel, and therefore we require the nazir to come up to the Land of Israel, so that he will count his nezirus in a state of purity, complete it, and bring his offerings. The question is how many days of nezirus he is obligated to keep in the Land of Israel, since we are speaking of a long nezirus that could last an entire year. It is worth emphasizing that the tumah he contracted outside the Land is only tumah of rabbinic origin. What, then, is required of him from the moment he enters the Land of Israel?
The dispute between Beis Shammai and Beis Hillel:
Beis Shammai - "nazir sheloshim yom": Once he becomes pure, he is a nazir for only thirty days. Since the tumah is only rabbinic, the Sages required of him no more than thirty days. Had we regarded his stay outside the Land as full-fledged tumah, it would seem that he would have to begin everything anew - especially since we are dealing with a nezirus longer than thirty days.
Beis Hillel - "nazir batechilah": One must be stringent, and he must count his nezirus over again from the beginning.
The story of Queen Helene:
The mishnah brings a story that supports the position of Beis Hillel: the son of Queen Helene went out to war, and she said, by way of a vow and a plea for merit: "Im yavo beni min hamilchamah beshalom, eheyeh nezirah sheva shanim" - "If my son returns from the war in peace, I will be a nezirah for seven years." Her son returned from the war in peace, and she was a nezirah for seven years. At the end of the seven years she came up to the Land of Israel to complete her nezirus, "vehoruha Beis Hillel shetehei nezirah od sheva shanim acheiros" - and Beis Hillel ruled for her that she must be a nezirah for another seven years, in accordance with their position that the count returns to its beginning.
And at the end of those seven years she became tamei, so that she turned out to be a nezirah for twenty-one years: seven years outside the Land, seven years in the Land of Israel, and once she became tamei - another seven years.
The opinion of Rabbi Yehudah:
Rabbi Yehudah said: "Lo haysah nezirah ela arba esreh shanah" - that is not how the story happened, and she was a nezirah for only fourteen years, not twenty-one.
Two ways of understanding Rabbi Yehudah's words:
Some understand that she never became tamei at all, and that part of the story is simply not accurate. She did not become tamei at the end of her nezirus; rather, she kept an additional seven years when she came up to the Land of Israel, and with that her nezirus was completed.
Others understand that she did indeed become tamei, but that she became tamei on the very last day of her nezirus. The mishnah follows the opinion of Rabbi Eliezer that we saw earlier, that a nazir who becomes tamei on the last day of his nezirus is a nazir for only thirty days. Therefore, even though she became tamei at the end, she had to keep only thirty days. According to this understanding, although Rabbi Yehudah says "fourteen years," he means fourteen years and thirty days, only that he did not mention the thirty days.