How to convert my macro to send emails automatically

I have attached a sample workbook.
The workbook already has a macro that can send an email after working out if the date is overdue.
I have put a button on the screen to help with this, but I would like to remove the button and automate it, however, there is one problem, I haven't a clue how to do it.
I have looked at so many posts about this I can't remember where I got the initial macro from,( thank you to whoever initially put it together) but I have since made some changes to it regarding how the message displays etc.
Could someone advise me please? also whilst looking at it, could anyone tell me what i, 15 or i, 16 in the macro means, There isn't anything in column i and I can't work out the relevance of it.
Thank you

This will automatically run your macro when you Open the workbook. Put this code in the ThisWorkbook code module.

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.Cells(i, 15) refers to a Cell's (row, column) where i is the row number and 15 is the column number. The i is a variable that will loop from 2 to the last used row. The 15 (in this case) is column O.