Publicizing the victory[1]
ואחר-כן באו בניך לדביר ביתך ופנו את-היכלך וטהרו את-מקדשך והדליקו נרות בחצרות קדשך וקבעו שמונת ימי חנוכה אלו להודות ולהלל לשמך הגדול
Afterwards Your children went to your Holy abode, cleaned out Your Heichal, purified Your Sanctuary, and lit candles in Your Holy courtyard. They established these eight days of Chanukah for praising and thanking Your great Name[2]
Seemingly the first day of Chanukah is no different than the other days of Chanukah. However, the Pri Chadash notes[3] that in the original Chanukah story, the first day seemingly didn’t contain any miracle. They found a jug of oil which was enough to last one day. The fact that it lit for one day isn’t a miracle. He concludes then that the reason why we celebrate eight days and not seven is that the first day commemorates the miraculous victory of the tiny Jewish forces against the vast Greek Empire. The Jews reclaimed the Temple and were able rebuild the destroyed Menorah and light it.