Carry out the following steps to create a formula field for opportunity records to derive the week number for the Opportunity Close Date field:

1. Click on the Setup gear icon in the top right-hand corner of the main Home page, as shown in the following screenshot:

2. Click on Setup, as shown in the following screenshot:

3. Navigate to the Opportunity customization setup page as follows:
   Objects and Fields | Object Manager | Opportunity | Fields & Relationships.

Locate the Fields & Relationships section on the right of the page.

4. Click on New.

We will be presented with the Step 1. Choose the field type page.

5. Select the Formula option.
6. Click on Next.

We will be presented with the Step 2. Choose output type page.

7. Enter Close Date Week Number in the Field Label textbox (as shown in the upcoming screenshot).
8. Click on the Field Name. When clicking out of the Field Label textbox, the Field Name is automatically filled with the value Close_Date_Week_Number.
9. Set the Formula Return Type as Number.
10. Set Decimal Places to 0 in the Options section, as shown in the following screenshot:

11. Click on Next.

We will be presented with the Step 3. Enter formula page.

12. Paste or enter the following code in the formula editor box:

/*********************************************************** 

Week Number Formula 

- 2017,01,01 : Jan 1 2017 : Sunday (used as a reference point) 

- Calculate difference between reference point and Close Date (no. of days) 

- Divide this number of days range into weeks ( / 7 ) 

- FLOOR function removes any decimal places to give whole no. of weeks 
- MOD function give modulus of 52 (a year) to return the remainder weeks 
- Finally the calculation is incremented by 1 to give starting week no.  
***********************************************************/ 
MOD(FLOOR( ( CloseDate - DATE(2017,01,01) ) / 7), 52 ) + 1 

13. Optionally, enter details in the Description field.
14. Optionally, enter details in the Help Text field.
15. In the Blank Field Handling section, leave the setting as the default option Treat blank fields as zeroes.
16. Click on Next, as shown in the following screenshot:

We will be presented with the Step 4. Establish field-level security page.

17. Select the profiles to which you want to grant read access to this field via field-level security. The field will be hidden from all profiles if you do not add it to field-level security.
18. Click on Next.

We will be presented with the Step 5. Add to page layouts page.

19. Select the page layouts that should include this field. The field will be added as the last field in the first two-column section of these page layouts. The field will not appear on any pages if you do not select a layout.
20. Finally, click on Save.