How to pass a variable value from a Userform listbox/spinbutton to another sub

Sorry but this is an incomplete description:

under the "OK" button I increase the selected value by 1, so if they've clicked on January it stores "1",

Increase what selected value? Stores it where?

naming the varaibles public

What variables? You haven't said anything about variables in your description.

Based on what you have described, there are two ways to do this. The simplest is to refer to the listbox and textbox in the sub. You don't give the name of your form so I'm going to call it UserForm1, and so on for the other objects.

' Code in Module1

Dim UserMonth As Long
Dim UserYear As Long

UserMonth = UserForm1.ListBox1.ListIndex + 1 ' 1 = January, 12 = December
UserYear = UserForm1.TextBox1

The other more complicated way is to declare public variables in your UserForm code, and assign them as above when the user clicks OK. Then refer to those variables in the other Sub. This breaks the dependency so that Module1 no longer has to know how UserForm1 has implemented the controls (which is an advantage).

' Code in UserForm1

Public UserMonth As Long
Public UserYear As Long

Private Sub CommandButtonOK_Click()

   UserMonth = ListBox1.ListIndex + 1 ' 1 = January, 12 = December
   UserYear = TextBox1

End Sub

' Code in Module 1

Dim UserMonthLocal As Long
Dim UserYearLocal As Long

UserMonthLocal = UserForm1.UserMonth  ' 1 = January, 12 = December
UserYearLocal = UserForm1.UserYear

Why not open the workbook(s) with code in the userform?