=WEEKDAY returns a Day No. for a particular date, based on which day the date is falling on.
 
Example, =WEEKDAY (A1, 2), where A1 is a Monday, the output of the formula will be 1.
The parameter that you choose is extremely important. Please see below table.
PARAMETER
NUMBER RETURNED
1 or omitted
Numbers 1 (Sunday) through 7 (Saturday).
2
Numbers 1 (Monday) through 7 (Sunday).
3
Numbers 0 (Monday) through 6 (Sunday).
11
Numbers 1 (Monday) through 7 (Sunday).
12
Numbers 1 (Tuesday) through 7 (Monday).
13
Numbers 1 (Wednesday) through 7 (Tuesday).
14
Numbers 1 (Thursday) through 7 (Wednesday).
15
Numbers 1 (Friday) through 7 (Thursday).
16
Numbers 1 (Saturday) through 7 (Friday).
17
Numbers 1 (Sunday) through 7 (Saturday).
 
I usually use ,2 as it helps to me work on weekdays & weekends.
With ,2 all weekdays lie between 1 to 5 and all weekends are 6 to 7.

Please see below example of how you can actually apply this.

The date 15-02-2014 is a Saturday. I have found this out using the =TEXT formula.
Click here to know about more the =text formula
 
For the Day No., I have used the formula =WEEKDAY(A2,2)
This gives me 6 as output for Satuday, as explained above.
 
Then I have simply used an =IF formula to find out whether it is a WeekDAY or a WeekEND
=IF(WEEKDAY(A2,2)>5,”WeekEND”,”WeekDAY”)