for more details refer to RFC5545 ALARM component, in your case the resulting icalendar file would look like:
BEGIN:VCALENDAR
VERSION:2.0
PRODID:pyICSParser
BEGIN:VEVENT
DTSTART:20130214T190000
DTEND:20130214T190000
URL:http://www.myurl.com
DTSTAMP:20130205T114500
UID:UID
CREATED:20130214T190000
DESCRIPTION:Wedstrijd":" 3000H5B2 CE\, Datum":" donderdag 14 februari -
19.00
BEGIN:VALARM
TRIGGER:-PT24H
REPEAT:1
DURATION:PT15M
ACTION:DISPLAY
DESCRIPTION:Reminder
END:VALARM
END:VEVENT
END:VCALENDAR