Holiday Accomodation

I app**** for a holiday accommodation for the weekend of 21-18 May on the 10th,last week. On the 12th i realized that we have a long weekend in April the 27th and tried changing dates just to realize that Holsboer was fully booked for that weekend. I sent through a request for cancellation before 08h00 in the morning of the 12th. I received response on the 13th that i will be liable for 100% cancellation fee as my cancellation is within 6 weeks. I do understand that they have T and C's but my concern is that as a client i need to be given as well a grace period to do amendments to my booking if required. I have always made my bookings with Holsboer and this is the 1st time i have had to cancel my stay. Secondly,if they had available accommodation for the 27th-30th April,i could have just rectified the dates. What was supposed to have been a great weekend away,is more of a night mare to me now. HelloPeter please intervene.