Earliest source for "Zos Chanuka"

Question

What is the earliest source/authority to ascribe special significance to the eighth day of Chanuka?

Answer 1

Thanks to all of the above commenters.

A Bar Ilan search shows that the earliest mention of the day known as "Zos Chanukah" is in the Sefer Haminhagim of R' Isaac Tyrnau, 14th-15th century (Minhagim Chodesh Teves). It doesn't specifically mention forgiveness, but it does make it sound like it is some sort of significant day (here's a link to a 1591 printing of it):

ובספר תורה שנייה קורין למפטיר בנשיא גמליאל וכו' [ומקצת בהעלותך] עד כן עשה את המנורה, כי יום שמיני הוא זאת חנוכה.

and later:

חתונה ומילה היו בשבת שני דחנוכה שהוא זאת חנוכה

Various later sources ascribing it Kabbalistic significance have been found (see links in comments), but they only begin in the 16th century.