How do I add a single number counting upwards from 1 into an email?

I’m writing a script that will send an email at a specified time every day. The body of the email must contain “day X” where X is the number of days the email has been sent. So the first email is day 1, the second is day 2, and so on. How do I do this without altering the script code every day to output the new value?