How do I modify the calendar so it starts with Monday ?

In order to change the day the calendar field start from you will need to edit the front-end language file located in the /language/en-GB/ folder. Just search for:

RSFP_CALENDAR_START_WEEKDAY="0"

The default calendar start day is Sunday and the count of days starts from 0, thus in order to change the start day to Monday you will only need to change the value of the placeholder to 1. Instead of adjusting the file, you should use the standard Joomla! language override feature to provide a different translation using the RSFP_CALENDAR_START_WEEKDAY language constant.