The first day of the month, the popular internet mailing list software package MailMan emails out a reminder to all subscribers on the list. The reminder contains the following text:

This is a reminder, sent out once a month, about your -domain- mailing list memberships. It includes your subscription info and how to use it to change it or unsubscribe from a list.

Some internet users are often subscribed to large numbers of mailing lists, this results in a deluge of mail reminders on the first of the month.