On my sidebar on desktop I have a clock, but the list of gadgets wont let me put a calender on ,it just appears blank can anyone tell me whats the likely cause
Unless you have a calender on your page of listed sidebar items, you will need to download one; as indicated by wildwood.
If by chance you do have one but it will not transfer by normal means to the sidebar, try clicking onto it and dragging and dropping it into position. In fact, I am able to drag and place sidebar items anywhere on my desktop's screen. ( I operate on Vista but I think it will also work with Windows 7 and possibly others.)