Anor was the name for the Sun in the Sindarin language, and so Oranor was one of the names given by the Elves to their day of the Sun. This was the second of the six days that made up the Elves' week, lying between Orgilion and Orithil. In Quenya the same day was called Anarya. After the Elves' day-names found their way to Middle-earth, the Hobbits used forms translated into the Common Tongue, so ultimately their week, too, had its own 'Sunday'.
Notes
1
If it were a simple Elvish word, the stress in Oranor would go on the first syllable, so the pronunciation would be 'o'ranorr'. However, this is part of the sequence of the days of the week, formed as a compound word from or anor, 'day of the Sun' . For these reasons, the pronunciation guide shown above places the stress on the second syllable. The 'rr' in the pronunciation guide indicates that the final 'r' sound of the word should be distinctly pronounced (as opposed to something like 'oranaw', which would be the more natural pronunciation for many English speakers).