Excel uses a serial number system to represent dates and times. January 1, 1900 is arbitrarily assigned the number 1. Time is expressed with decimal points - noon, the midway point during the day, is assigned 0.5. So the date 12/31/16 5:00 PM . . .
. . . would be represented as 42735.7083333333 - 5 PM being a little bit more than 70% way through the day, and December 31, 2016 being 42,735 days since the beginning of the 20th century.
If you already have a date entered in a cell the YEAR, MONTH, and DAY functions can be used to extract one of these values.
The WEEKDAY function will indicate the day of the week as numeric value. If you reference a date followed by a 1, Sunday will be counted as the first day; followed by a 2, Monday will be counted as the first day. A 3 given as the 'return type' will count Monday as the first day but start at 0.