Meant to send regular reminder emails to i.e. students, this is a very basic script. The options aren't too complicated. Each row of the spreadsheet controls an email that can be send at a certain time each week or each month (day of week or day of month timers).
The goal is to keep the code and the script very simple. If you need more complicated rules such as something send multiple times a week or month or what have you, you can just fill in multiple rows of the spreadsheet. In this way, basic spreadsheet functionality multiplies the capability of a very simple tool.