גמ׳ אמר רב חנן בר רבא קלנדא ח' ימים אחר תקופה סטרנורא ח' ימים לפני תקופה וסימנך (תהלים קלט, ה) אחור וקדם צרתני וגו' ת"ר לפי שראה אדם הראשון יום שמתמעט והולך אמר אוי לי שמא בשביל שסרחתי עולם חשוך בעדי וחוזר לתוהו ובוהו וזו היא מיתה שנקנסה עלי מן השמים עמד וישב ח' ימים בתענית [ובתפלה] כיון שראה תקופת טבת וראה יום שמאריך והולך אמר מנהגו של עולם הוא הלך ועשה שמונה ימים טובים לשנה האחרת עשאן לאלו ולאלו ימים טובים
Rav Hanan bar Rava said: Kalenda is celebrated in the eight days after the Winter solstice. Saturnalia is celebrated in the eight days before the Winter solstice.
The Sages taught: When Adam, the first human, saw that the days were progressively getting shorter, he said, "Woe is me! Perhaps it is because I sinned that the world is becoming dark around me, and returning to chaos and void. And this is the death that is sentenced upon me by heaven." He arose and sat eight days in fast and prayer.
When he saw the time of Tevet (the solstice) and saw that the days were getting progressively longer, he said, "This is the way of the world." He went and made a festival for eight days. The next year he observed both (the days he had fasted and the days he had celebrated) as a festival.
The First Havdalah
A midrash tells us that on the first Shabbat, the sun shone for 36 hours. When the sun was about to go down on Friday night, God decided to honor Shabbat by letting the light continue to shine throughout Shabbat. When the sun set at the end of Shabbat, the darkness began to set in. Adam was terrified (having been created the day before and never having experienced darkness) thinking, surely the darkness will envelop me. What did God do for him? He gave him two flints which he struck against one another. Light appeared and Adam uttered a blessing on it. And this is why we bless the light at Havdalah.
-Bereishit Rabbah 11:2