Westwood wrote on Jan 12^th, 2012 at 10:59pm:

The site will not let you go more than 5 months into the future. Westwood

5 months is the rule for all reservations on OntPPs.
There are also penalties for cancelling/changing 4 months or less before the entry date.  I think they get punitively higher the closer to your entry date you make the change.

People used to book their favourite sites a year in advance and for the maximum stay allowed, cherry-picking the best sites.  Then they wouldn't use the reservation.  The site goes empty.  The park doesn't collect their cash.  People who would have used the site can't.  Basically it became a de-facto case of if you could afford it you could claim a site and have it go vacant.  In a fit of rightous egalitarianism (justly so imo) restrictions & penalties were put in place with the result being more people using the parks and 'offenders' paying the price for their campsite-camping ways.

What's really fun is trying to book a yurt.  They go fast.