Scottish Equi-Ads seems to have most of the dates of events. And it's free every month. Just need to stop shopping cos I always spend money at the same time as getting a copy.
If you compete within a specific discipline then you kind of know what competitions are held and where. Central Scotland affiliated dressage is probably going to be Kingsbarn, SNEC, Rowallan or Ingliston, again Riding clubs have their own websites and you tend to know locally what facilities the riding clubs use.
Is there something/somewhere in particular you are looking for?
as people have said, scottish northern equestrian, local rc's website some yards have their own page aswell, equi ads has a list of dates, also use the area riding club website as well thats where i get most of mine from