It is an ancient tradition for the Chatan Torah and Chatan Bereshit (the bridegrooms of the law), who are called up for the last and first aliyot of the annual Torah reading cycle, to give a kiddush. The earlier custom was to provide a whole feast and to do it on Simchat Torah. Now it is generally done on Shabbat Bereshit. The chatanim thereby share with the community their joy on taking part in completing the Torah.
This custom has been controversial. Some halachic writers observed that it all too often led to people “pushing and shoving each other so that the joy turns to grief” (not a sight I can ever recall seeing around the whisky and herring in any British shul). When some communities cancelled the feast of the chatanim, the Mishnah Berurah, in 19th century Poland, protested. He wrote that there were many who celebrate all the rest of the time, so that their every day was like a holiday. How then can we not also eat and drink to celebrate finishing the Torah?