Formula calculating calendar weeks (2015 vs 2016)

Here is a dev forum link on calculating the week no. The solution is for all years. Hope this helps.

Extracts from the link:

To be able to do weekly comparison report Year-to-Year with this weeknumbering it is necessary to create custom fields for the opportunity that calculates the Year and Week number for a given close date. Here is one solution using three custom formula fields.

Field: Global Sales Report Weekday
Description:  Day 1 = Sunday, 2 = Monday....., 7=Saturday
Formula:  MOD( CloseDate  - DATE(1900, 1, 7), 7)+1

Field: Global Sales Report Week Year
Formula: YEAR(CloseDate + (MOD(8- Global_Sales_Report_Weekday__c ,7)-3))

Field: Global Sales Report Week
Formula:
FLOOR((CloseDate - DATE( Global_Sales_Report_Week_Year__c ,1,1) + MOD((MOD( DATE(Global_Sales_Report_Week_Year__c,1,1) - DATE(1900, 1, 7), 7)+1)+1,7)-3) / 7 + 1)