ical_splitter.rb

# gem install icalendar
require 'icalendar'

# Open a file or pass a string to the parser
cal_file = File.open("/path/to/my_large_calendar.ics")

# Parser returns an array of calendars because a single file
# can have multiple calendars.
cals = Icalendar::Calendar.parse(cal_file)
cal = cals.first


event_slices = cal.events.each_slice(500).to_a

event_slices.each_with_index do |event_slice, idx|
  cal.events.replace(event_slice)
  File.open("events#{idx}.ics","w") {|f| f.write cal.to_ical}
end