You can't have more than 256 columns. You can split your calendar into
two lots of  6 months and then keep these on two separate sheets.

Alternatively, arrange your dates to go down the sheet - you could then
have 65,000+ of them. If you want to do this and have already entered
some dates, then check out "Transpose" in Excel Help as a way of moving
the data through 90 degrees.

Hope this helps.

Pete