I agree totally with the 2 previous posts, but some additional information.
People write and speak a specficic date in different ways, e.g.
15 October (not too common)
the 15th of October
15th of October.
The choice of which to use is partly BE/AE, partly regional within countries and some are considered more formal or businesslike.
There are also some special times of year where, in BE at least, we use at. e.g. Christmas, New Year and Easter. We say at Christmas or at the New Year, but on Christmas Day, etc.