If you have been given a week number and has been asked to find the first Sunday for that week, you can use following formula
Where A1 has the year say A1=2016
5 is the Week Number which you can replace.
For Finding Monday, add 1 in the formula, add 2 for Tuesday and so on.
The above formula assumes that WEEKNUM function has Sunday as the starting day for the week. If you have any other day for the week as the starting day of the week, you will have to customize the above formula as per the need.