YAUM- Lexicography
Yaum: A day of 24 hours ( Plural `Ayyaam ' - In contradiction to 'Lail wWa Nahaar' - 'Night and Day ). (A Dictionary of Islam, Thomas Patrick Hughes)

Some derivatives and shades of meaning of the root word "Yawm, Yaam" have been given in the Arabic-English Dictionary by J.G.Hava, as follows:

'Yawm' , 'Yaam': To spend, to last a day, distressing day, distressing long day, the last day of the month.
Yaumun: Day, Time periods (s). Commonly it means time period from sunrise to sunset . In another sense it covers 24 hours time from sunrise till the next sunrise (i.e day and night, both ).
It also means a period of time or a particular time- It is a measure of time.
(Lughatul Quran, M.Abdur-Rashid Nomani).

"Allah has preceded the word Ad-Diin by Yaum (day) and not by night, as it is during day that we can differentiate between different objects. This is fixed and is unalterable, though its knowledge is confined to Allah Alone.” (Page 29, Vol-I,A.M.Yaqoob).

Yaum (Al-Yaum) and related usage may be seen in pages 882-888, Al-Muajamul Mufahras, Muhammad Fawad Abdul Baqi.