Gemara

4a - Beraisa: If one vowed a Neder, Hekdesh or Erchin, once three Regalim pass (if he did not fulfill it) he transgresses Bal Te'acher;

R. Shimon says, they must be in order, starting with Pesach;

He transgresses after three Regalim if he vowed before Pesach, after five Regalim if he vowed before Shavu'os, or after four Regalim if he vowed before Sukos.

Beraisa: After three Regalim one is liable for not fulfilling a vow of Damim, Erchin, Cherem, Hekdesh, Chatas, Asham, Olah, Shelamim, Tzedakah, Ma'aser (of produce), Bechor, Ma'aser (Behemah), Pesach, Leket, Shichchah or Pe'ah.

R. Shimon says, he is liable after three Regalim in order, starting with Pesach;

R. Meir says, he is liable after one Regel;

R. Eliezer ben Yakov says, he is liable after two Regalim;

R. Elazar b'Rebbi Shimon says, he is liable after Sukos.

The first Tana learns from the repetition of "B'Chag ha'Matzos uv'Chag..." The Regalim were listed above, so the repetition teaches about the Shi'ur for Bal Te'acher;

R. Shimon says, even though the Torah repeats the Regalim to teach about Bal Te'acher, it did not need to repeat Sukos, for it was mentioned last. It was repeated to teach that it is the last (Regel before one transgresses Bal Te'acher).

R. Meir learns from "U'Vosa Shamah; Va'Haveisem Shamah";

Chachamim disagree, for this is only an Aseh;

R. Meir says that since he transgresses the obligation to bring it, automatically he transgresses Bal Te'acher.

R. Eliezer ben Yakov learns from "Eleh Ta'asu la'Shem b'Mo'adeichem" - the plural teaches about at least two;

Chachamim use this like R. Yonah, to teach that the Se'irei Chatas of Rosh Chodesh and festivals atone for the same matter, i.e. Tum'as Mikdash and Kodoshav.

R. Elazar b'Rebbi Shimon holds that the Torah did not need to repeat Sukos. It was repeated to teach that Bal Te'acher depends on it alone.

6b - Rava: Once three Regalim have passed, one transgresses Bal Te'acher every day.

Question (Beraisa): Bechor is like all Kodshim. Once a year has passed without (three) Regalim, or Regalim without a year, he transgresses Bal Te'acher.

Question: How does this refute Rava?

Answer (Rav Kahana): The Tana seeks all the Lavim possible, and he does not mention Rava's law.

Answer: The one who asked Question (i) (and, presumably, also Rava) holds that the Tana merely teaches when one first transgresses.

Question: The Beraisa says that one transgresses after a year without Regalim. How is this possible?

Granted, it is possible according to R. Shimon, who requires the Regalim to be in order. How is it possible according to Chachamim?

Answer #1: It can be like Rebbi, who says that a year is 365 days. In a leap year, this is (19 days) less than a year from day to (the same) day (of the same month).

Answer #2: It can be like Chachamim of Rebbi (who say that a year is 12 months, or 13 months in a leap year). The Tana holds (unlike Acherim) that Shavu'os can be on the fifth, sixth or seventh of Sivan. (A year from Sivan 6 to Sivan 6 need not include Shavu'os.)

7b - Beraisa: The 16th of Nisan is Rosh Hashanah for the Omer. The sixth of Sivan is Rosh Hashanah for Shtei ha'Lechem.

Question: There are more than four Roshei Hashanah!

Answer (Rav Shisha brei d'Rav Idi): The Mishnah lists only those which do not depend on an action (such as Hakravah).

Question: The Mishnah lists Regalim (for Bal Te'acher. One is liable once he could have brought his Korban, i.e. after the Tamid was offered!)

Answer: No, Bal Te'acher takes effect automatically (from the start of the day).

Rishonim

Rif and Rosh (1:1): If one vowed a Neder, Hekdesh or Erchin, once three Regalim pass (if he did not fulfill it) he transgresses Bal Te'acher. R. Shimon says, they must be in order, starting with Pesach. "Motza Sefasecha" is an Aseh (to fulfill Nedarim in time); "Tishmor" is a Lav; "V'Osisa" teaches that Beis Din forces people to fulfill their Nedarim; "Ka'asher Nodarta" is a Neder; "la'Shem" is Chatas, Asham, Olah and Shelamim; "Nedavah" is Nedavah (one says 'this is a Korban'); "Asher Dibarta" are Kodshei Bedek ha'Bayis; "b'Ficha" is Tzedakah.

Rambam (Hilchos Pesulei ha'Mukdashim 14:13): It is a Mitzvas Aseh to fulfill Nedarim, Nedavos and other obligations to pay Erchin, Damim, Ma'aseros and Matanos Aniyim at the first Regel. We learn from "U'Vosa Shamah; Va'Haveisem Shamah". When you come to Yerushalayim for the Regel, you must bring all your obligations. If he did not bring his obligation at the time of the Regel, he was Mevatel the Aseh. If three Regalim passed and he did not offer the Korbanos or pay the money he transgressed "Lo Se'acher Leshalmo". There are no lashed for this for there is no action.

Ri Korkus and Kesef Mishneh: Even though the Stam Mishnah is like R. Shimon, the Rambam is not concerned for this because many Tana'im argue with him in a Beraisa. Therefore, he adopted the medium Shi'ur. Also Rava and a Stam Sifra hold that one is liable after three Regalim. Also, the purpose of our Mishnah was to teach about Roshei Hashanah, and not about Bal Te'acher.

Mishneh l'Melech: I know of no source for an Aseh for anything not offered in the Mikdash. Rashi explains that 'Terumas Yedchem' is Bikurim. The Rambam holds that the Aseh applies even nowadays. Sefer ha'Chinuch holds that it is only when the Mikdash stands. One is not liable until three full Regalim pass. If one vowed in the middle of the Regel, the last half of the Regel joins with the first half the following year (Tosfos 6b DH Sof).

Question: The Gemara said that the Beraisa that obligates after a year without Regalim can be like R. Shimon. But R. Shimon said that one can be liable after three, four or five Regalim. If one is liable after a year, he is always liable before four or five Regalim!

Answer #1 (Rashi 6b DH ha'Nicha): The Tana holds like R. Shimon in one respect (the Regalim must be in order), but he argues in one respect (this Tana says that one is always liable after three Regalim).

Answers #2,3 (Ba'al ha'Ma'or): The Tana holds exactly like R. Shimon. One transgresses (a Lav derived from an Aseh,) "Sochalenu Shanah v'Shanah" after one year, but he does not transgress Bal Te'acher until the Regalim pass in order. Alternatively, he transgresses Bal Te'acher after a year, but he is not liable for every day until the Regalim pass in order.

Rashba (6b DH Ha): We could have answered that a year can pass without Regalim if one was sick on the Regel.