I’ve long read where the re-dedication/dedication of the Temple after it was defiled by Antiochus required eight days. Where does that law come from?
The eight day period was not originally part of the law. There are several opinions about the origin of the eight days, but here’s one I found in several unrelated sources. (A thing is established on the testimony of two or three witnesses Deut. 19:15.)
The Menorah in the Temple had to burn every day and sacred oil was stored in one day containers, sealed by the High priest for that purpose. After the Temple was recaptured and repaired and the Jews went to re-light the Menorah, they could only find one container of sacred oil that hadn’t been ruined.
It would take time to make more oil, but rather than delay, the priests decided to use the one day supply they had and hope the Lord would find it acceptable. They poured the oil into the menorah, but the Lord miraculously refilled the container giving them another day’s supply. This happened each day until the new supply of oil was ready, eight days later. Then it stopped and the priests began to use the new supply.
The Jewish leaders decreed that a special festival should be observed to commemorate the miracle of the oil, and that it should last eight days, one for each day of miraculous provision. That’s why Hanukkah is eight days long.