This is probably a very stupid question, but.

My lesson falls on Halloween this year, the stable will be open then correct?

Most places(business/stores etc..) at least around my area do not close due to Halloween , but if you are in doubt , double check with the stable owners/managers and then they can give you the hours pertaining to their stable. Maybe they will have a fun Halloween theme and you can dress up or do games with your horse...bobbing for apples would be a fun horse related game : ) My horses would love that one!

Around where I live, nothing closes due to Halloween.