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