As mr p will confirm, advertising should be a constant thing you do, rather than just when you are slow. As it takes time to draw clients/customers in.
We run a sign business as well, and when we get told by people they don't need to advertise or its all through word of mouth, that's good, but things change, people die, move area or just plain stop requiring your services, they don't give you warning.
Advertising should be constant, if you are full you just say sorry you can't fit them in or it'll be in 4 weeks not 2. Customers coming to your door is not a button you can turn off or on when things are slow or busy.
Just my opinion xxxx