What the text says. John gives both the feast and the season: "And it was the feast of the dedication at Jerusalem: it was winter; and Jesus was walking in the temple in Solomon's porch" (John 10:22-23, ASV).
Where it came from, and why it is not in the Law. The seven annual feasts of Leviticus 23 do not include it, and they could not: the events it commemorates happened long after. In 167 BC Antiochus IV Epiphanes desecrated the Jerusalem Temple; the Maccabean revolt recovered it, and the altar was cleansed and rededicated. The 1915 ISBE describes the resulting feast: "A feast held by the Jews throughout the country for eight days, commencing on the 25th Kiclev (December), in commemoration of the cleansing of the temple and dedication of the altar by Judas Maccabeus after their desecration by Antiochus Epiphanes" (citing 1 Maccabees 4:56, 59). It adds that the feast was to be kept "with mirth and gladness", that 2 Maccabees describes it kept like Tabernacles "with the carrying of palm and other branches, and the singing of psalms", and that Josephus calls it "Lights", from the joy which accompanied it (Antiquities 12.7.7).
Why the porch matters. Kislev falls in December, and John says so. Solomon's porch was a covered colonnade along the eastern side of the Temple's outer court, which is a plain practical reason for a teacher and a crowd to be standing there in that month, and an example of a detail that reads as incidental until the calendar is supplied. ⚠️ Note that the sources for the feast's origin are 1 and 2 Maccabees, books held as Scripture by Roman Catholic and Orthodox Christians and as valuable but non-canonical history by Jews and Protestants. That difference of status is real and is stated here rather than assumed either way; the historical events they describe are not themselves in dispute.
Sources: John 10:22-23; 1 Maccabees 4:56-59; 2 Maccabees 10:6-7; Josephus, Antiquities 12.7.7; ISBE 1915, "Dedication, Feast of".
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