There is no definative date Steve, they stay open until the snow is gone. There have been years that skiing has been possible well into May but to extend holidays past mid March would be too big a gamble for tour operators to take.
Hi Pete. thanks for the quick reply.
Our friend have just brought an appartment in Bansko, and where thinking of taking us there on the 9th of April. I know that most resorts have fixed dates for open and closing. Do you know what these are in Bansko, or where I can find this info. Google has failed.