Long term touring - weekends

Hi,

we will be picking up our new (to us) Caravan this Saturday and intend to spend most of the summer touring in the U.K. I have already noticed many weekends at some sites are already booked for most of the summer, but how likely is it that there will be vacancies at these sites nearer the time. For example if we booked the week days is it likely that by the time the weekend came there would be vacancies

Thanks for any info