terminal linux calendar specify used calendar file

I´m playing around with linux calendar

My aim is that all users can run a script which does nothing more than adding a givven argument in 10 days as cal item

// called like this: reminder "Meeting with Frank" 
// will add a calendar item in 10 days


#!/bin/bash

remdate=$(date +"%m/%d"  -d "10 days")
echo -e "$remdate\t$1" >> /etc/.calendar/calendar

All users have permissions for /etc/.calendar/calendar

The calendar will run by cron to inform about todays meetings:

calendar|mail -s "do not forget" xy@example.com
or 
calendar -f /etc/.calendar/calendar |mail -s "do not forget" xy@example.com

But how do I tell calendar to not use the users calendar file (~/.calendar) or in /usr/share/calendar but the one I´m writing to? (tried -f and I´m confused with /usr/share/calendar and CALENDAR_DIR)

thanks