Gemara

In Eretz Yisrael, since all ascend to Yerushalayim on the festivals, witnesses can be found to verify the signatures on a Get.

Question: This applies only when the Mikdash stands. How can we answer for after the Churban?

Answer: Since Batei Dinin are fixed, people regularly travel from county to county, and witnesses are available.

Ta'anis 10a (Mishnah - R. Gamliel): We start to request rain on the Cheshvan 7, so the slowest travelers (who came for Sukos) will reach the Pras river.

(Beraisa - Chananyah): In Bavel we begin 60 days after the fall equinox.

4b (Rav Asi citing R. Yochanan): The Halachah follows R. Yehudah (we mention rain in Musaf of Shemini Atzeres, but not in Shacharis).

Question (R. Zeira): R. Elazar said that the Halachah follows R. Gamliel (we do not request until the seventh of Cheshvan)!

Answer #1 (Rav Asi): R. Yochanan argues with R. Elazar.

Answer #2: R. Yochanan discusses Eretz Yisrael; R. Elazar discusses Bavel.

Question: In Bavel we delay requesting, lest rain harm produce in the field. Also in Eretz Yisrael they should delay, due to returning travelers!

Answer: R. Yochanan discusses after the Churban (there are no travelers).

Answer #3: Both Amora'im discuss Eretz Yisrael. R. Elazar discusses when the Mikdash stands, and R. Yochanan discusses after the Churban.

Medrash Shir ha'Shirim (4:2 and 8:3): "You are eyes are doves" - even if one takes the chicks under a dove, it will never leave its dovecote. Similarly, even though the Mikdash was destroyed, Yisrael were not Mevatel three Regalim. "...We will draw on a cedar tablet" - even if a picture is erased, its impression remains. Similarly, even though the Mikdash was destroyed, Yisrael were not Mevatel their steps (to the Mikdash) three Regalim each year.

Rishonim

Rif and Rosh (Ta'anis 2a and 1:4): The Halachah follows R. Gamliel.

Rambam (Hilchos Tefilah 2:16): From Cheshvan 7 we begin to request rain in Eretz Yisrael. In Bavel, Surya, Mitzrayim and places close to them and similar to them, we begin on the 60th day after the equinox.

Ran (DH v'Ika): The Rif and Rambam do not mention produce in the fields; they rule like R. Gamliel. They hold that even today in Eretz Yisrael we wait until the seventh. Even today, people gather to go to Yerushalayim on the Regel; for their sake we delay requesting rain.

Tashbatz (3:201): The Kedushah of Eretz Yisrael ceased, but the Kedushah of the Mikdash and Yerushalayim persists. A support is that people are Oleh l'Regel (ascend to Yerushalayim for the festival) from Mitzrayim and other lands. There is a hint in the Medrash. The miracle that no one complains about being crowded remains. The Beis ha'Keneses in Yerushalayim is used the entire year. On Shavu'os many pilgrims come. The Beis ha'Keneses is filled from wall to wall with 300 people, yet everyone has space. This shows that the Kedushah persists.

Maharatz Chayos (Gitin 4b): We find that even after the Churban, people used to go to Yerushalayim during the Regel to ask questions where Batei Din were fixed, or because all the Chachamim used to gather.

Poskim

Shulchan Aruch (OC 117:1): In Eretz Yisrael we begin on Cheshvan 7.

Shulchan Aruch ha'Rav (1): Even after the Churban, places around Yerushalayim used to gather in Yerushalayim for the Regel, therefore they did not nullify the enactment to wait until Cheshvan 7 in Eretz Yisrael.

Kaftor v'Ferach (6 DH Perek): The Mitzvah to be Oleh l'Regel today is to increase pain over the Churban. Similarly, Chachamim permitted to crack nuts, etc on Yom Kipur after Minchah to increase pain of the fast (Shabbos 115a).

Chasam Sofer (YD 233 DH 'Ha): In the days of the Ge'onim, they used to go to Yerushalayim during the Regel. Perhaps one who lives far away is greater than one who lives in Yerushalayim, for he gets reward for walking there.

Yavi'a Omer (5 OC 15): Sefer Chasidim says that Rav Hai Gaon used to go from Bavel to Yerushalayim every Sukos, and circle around Har ha'Zeisim seven times on Hoshanah Rabah.

Chasam Sofer (YD 234 DH veha'Nir'eh): Nowadays there is no Mitzvah to go to Yerushalayim during the Regel. A man may not go without permission of his wife. Even Talmidim need permission to go away to learn! All the more so he needs permission (during the Regel) when there is a Mitzvah to make her happy! She cannot force him to go to Eretz Yisrael or Yerushalayim, even though she is Batel from the Mitzvah to live in Eretz Yisrael.

Teshuvah Kol Mevaser (2:10 DH Aval): There is a Mitzvah nowadays to be Oleh l'Regel, but not for Pesach Sheni, which is only for the sake of Korban Pesach Sheni.

Note: Kol Mevaser (2:43) and other Acharonim cite She'alas Ya'avetz (1:87), who brings the Medrash from Shir ha'Shirim, to say that Aliyah l'Regel is a Mitzvah nowadays. However, to explain why R. Gamliel was near the Mikdash after the Churban, the Ya'avetz says that he went to Me'aber the year, which must be done in Yehudah! (Yavneh is in Galil.)

Noda bi'Yehudah (2 OC 94 DH ume'Atah): The Tur and Shulchan Aruch omitted the Chiyuv to visit one's Rebbi during the Regel because this is only when the Mikdash stands and we are obligated to go to Mikdash during the Regel. Nowadays the Shechinah is still there, but the Mitzvah to go is only for the sake of Korbanos hence it does not apply today, so we do not obligate showing more honor to one's Rebbi than to Shamayim!

Tzitz Eliezer (10:1 93): Yisre'elim feel that Kedushas ha'Mikdash did not cease, therefore they go to the Mikdash even though there is no Mitzvah nowadays.

Yechavah Da'as (1:43 DH u'Milvad): One should also agonize over the spiritual state of Yerushalayim: tens of thousands of children are educated without Torah and Mitzvos, breaches of Tzni'us and Musar, Chilul Shabbos, violations of Kashrus...