A Quranic Journal: Surah al-Fajr ayah 3

Ash-shaf'i literally means "even" while al-watr means "odd". This has been interpreted in various ways: Watr is the Day of Arafat as it is odd (ninth day of the month) and Shaf'i is the day of Eid al-Adha as it is even (tenth day of the month). When Wasil ibn Sa'ib asked Ata whether the word watr used in the verse refers to the witr prayer, he said, "No, shaf'i is the Day of Arafat and watr is the night of Eid al-Adha."

...A hadith reported by Muslim and Bukhari says, "Allah has ninety-nine names. Whoever learns them by heart is one of the dwellers of Paradise." The hadith adds that Allah is witr (One) and He likes what stands alone... Some say that watr denotes Allah the All-Mighty and shaf'i signifies all the pairs such as heaven and earth, land and sea, human beings and jinn, sun and moon. Those who hold this view base it on the verse, "And of everything We have created pairs, that you may remember." (Surah adh-Dhariyat ayah 49).

Ibn Jarir did not believe that any of these views were conclusive