QUESTION: The Gemara expounds the verse, "How beautiful are your footsteps in sandals... daughter of the generous one" (Shir ha'Shirim 7:2), and says that the verse is praising the Jewish people who go up to Yerushalayim three times a year. The verse calls them "the daughter of the generous one" ("Bas Nediv"). "Nediv" refers to Avraham Avinu, who is called "Nediv" (c.f. Tehilim 47:10) because he was the first person to "convert," to generously dedicate his heart to recognize his Creator (RASHI).
Why is the praise, "Bas Nediv," applied to the Jewish people specifically when they are Oleh l'Regel? What is the relationship between the accolade of "the daughter of Avraham Avinu" and the Jewish people's ascent to Yerushalayim at the time of the festival? In addition, why does the verse refer to Avraham Avinu specifically in this context as "Nediv," the first to generously give his life to the service of Hash-m?
(a) The MAHARSHA explains why the Jewish people are praised as "the daughter of Avraham Avinu" when they are Oleh l'Regel. Avraham Avinu was not only the first convert, he was also the first to ascend to Har ha'Moriyah, the place of the Beis ha'Mikdash, in order to offer a Korban (his son, Yitzchak). The Jewish people follow in his ways when they ascend to Yerushalayim to offer Korbanos in the Beis ha'Mikdash during the festival.
The Maharsha does not explain why the Gemara refers specifically to Avraham Avinu as "the first of converts." The ETZ YOSEF explains that dedicating his life to serve Hash-m entailed for Avraham Avinu the necessity to leave his homeland and his father's household in order to travel to a distant land (c.f. Bereishis 12:1). The Jews who are Oleh l'Regel are similar to converts in that sense. They separate themselves from their households and leave everything behind in order to go to the Beis ha'Mikdash in Yerushalayim.
(b) The CHIDA (in Pesach Einayim) suggests that the words of the Gemara here are based on the Midrash (Bereishis Rabah 43:9). The Midrash teaches that as reward for his refusal to accept from Malki-Tzedek even "a thread (Chut) or a sandal-strap (Seroch Na'al)" (Bereishis 14:23), Avraham was blessed that his descendants would offer Korbanos on the Mizbe'ach (which was encircled with a "Chut," a painted strip of red dye) and would be given the Mitzvah of Aliyah l'Regel. The Midrash then quotes the verse cited by the Gemara here, "Ma Yafu... ba'Ne'alim" -- "How beautiful are your footsteps in sandals," an allusion to the reward that Avraham Avinu received for his refusal to accept even "a thread or a sandal-strap" from Malki-Tzedek.
(This answer explains the association of the Mitzvah of Aliyah l'Regel with Avraham Avinu, but it does not explain why Avraham is referred to as "the first of converts" specifically in this context.)
(b) The Gemara in Berachos (17b) relates that shortly before the festivals of Pesach and Sukos, the Jews in Bavel gathered together to learn Torah at the "Yarchei Kalah" in preparation for the festival. At that time, a great Kidush Hash-m occurred. When members of the foreign nations saw the immense glory of Hash-m as the Jews gathered together in a large assembly to learn Torah, they experienced a tremendous inspiration to come and convert.
This byproduct of the Jews' assemblage is mentioned in other Midrashim. The Midrash (Midrash Rabah to Shir ha'Shirim 1:15) says that the Jewish people are comparable to a Yonah (dove) in several ways (see Insights to Sukah 50:3). One way in which the Jewish people are comparable to a Yonah is that one type of Yonah, when fed, emits a scent that attracts other doves to its nest. Similarly, when the Chachamim teach Torah to the people, the nations who hear them come and convert.
In a similar vein, Rashi explains that in the blessing given to the tribe of Zevulun, "Amim Har Yikra'u" -- "Nations will gather at the mountain" (Devarim 33:19), "nations" refer to the Jewish people who gather at "the mountain" of Har ha'Moriyah and offer Korbanos to Hash-m during the festivals. The verse also refers to the nations of the world who travel to Eretz Yisrael to do business and who gather at Har ha'Moriyah where they witness the Jewish people serve Hash-m. The visiting nations become so impressed that they convert.
These two sources together imply that the nations come to Yerushalayim at the time when the Jews are Oleh l'Regel. At that time, the great assembly of Jews who perform the will of Hash-m in unison impresses the nations and inspires them to convert.
When the Jewish people conduct themselves in a way which inspires the nations to convert, they are referred to as "the daughter of Avraham," who was the first to convert and to inspire others to follow him (c.f. Bereishis 12:5). This is why the Gemara quotes the verse, "The noblemen of the nations gathered [to join] the nation of the G-d of Avraham" (Tehilim 47:10) -- they gathered in order to become part of the Jewish people (as the Maharsha explains) and to follow the ways of Avraham. Therefore, when the Jewish people are Oleh l'Regel and inspire the nations to convert, they are called "Bas Nediv," for they are following the ways of Avraham who led the way for converts. (M. Kornfeld. This approach is also proposed by RAV YOSEF SHAUL NATANSON in DIVREI SHAUL on Agados ha'Shas.)