One of the sources for celebrating finishing a Talmud tractate is in Kohelet Raba 1, 1 (or almost same in Shir Hashirim Rabah):
...,אָמַר רַבִּי יִצְחָק: חֲלוֹם עוֹמֵד עַל כַּנּוֹ
,"מִיָּד "וַיָּבוֹא יְרוּשָׁלָיִם ... וַיַּעַשׂ מִשְׁתֶּה לְכָל עֲבָדָיו
.אָמַר רַבִּי יִצְחָק מִכָּאן שֶׁעוֹשִׂין סְעוּדָה לְגָמְרָהּ שֶׁל תּוֹרָה
Says R' Yitzchak: The dream [of Solomon's prophecy] was immediately fulfilled... Immediately "he went to Jerusalem, ... and he made a feast for all of his servants." Says R' Yitzchak, from here we learn that one makes a feast on completing the Torah.
I understand that King Solomon was happy for the generous gift from Hashem and fulfillment of the prophecy, and made a great Thanksgiving feast.
What is "finishing Torah" in this statement and how does the celebration stem from Solomon's feast?