Zos Chanukah is the last day of the 8-day Jewish Chanukah Festival which celebrates survival against the Greek rule. Under Antiochus, the Greeks did not want to kill the Jewish people but banned Jewish study and practice. They banned: Jewish learning, Shabbos (the Sabbath) Milah, (circumcision), and Rosh Chodesh, (the Sanctification of the New Moon).
The last day of the 8 day Jewish Festival of Chanukah is called ZOS CHANUKAH. An explanation for this 8th day is because this is a day that’s beyond the laws of nature. The Maharal (Ner Mitzvah 2) teaches that the number seven represents nature and the number eight represents above nature. Chanukah is eight days, therefore all the days of Chanukah is a time for miracles. Zos Chanukah, which is the eighth day of the eight days of Chanukah enhances this uniqueness. On Zos Chanukah it is eight, and doubly beyond the rules of nature of this world.
The Maharal writes:
“The world of nature is subjected to the number seven, as the physical world was created in seven days. Therefore, whatever is beyond
nature is in the realm of eight… BRIS MILAH is beyond nature, since according to nature, a person was born with foreskin… Therefore
the milah is on the eight day… The Torah was also given after seven, as it states (Devarim 16:9) count seven weeks, and after the seven weeks, on the fiftieth day, the Torah was given… We already explained that a small jug of oil remained with the cohen gadol’s seal, and it’s known that the cohen gadol served with eight clothes. All of this is because of his level, that he had the level of eight [beyond nature]. Therefore the miracle was eight days.”