To create the drop-down list -- is it text or numerical values? -- you would first want to list out the variables somewhere. It could be another worksheet. For example, if your dropdown would offer a choice of:

Pizza
Tacos
Macaroni and Cheese
Chef Salad
Tofu Stir-Fry,

make a series of rows somewhere.

Then, where you want your dropdown, select "Data Validation" and choose the List option. Tell it your source data is the array of cells you typed the list in.

If your array is numbers or dates, then you can calculate based on the values selected.

Apply data validation to cells - Office Support

I'm not clear on whether your dropdown would BE the week number, or related to the week number in some way.