Start with the clients you already have, because they are the cheapest bookings available. Pull the list of people who came regularly and have not been in for longer than their usual gap, and message them individually. Not a broadcast — a short, specific note mentioning what you did last time. That single hour outperforms most advertising.
Then fill the gaps you can see. A last-minute space offered to your waitlist is money you would otherwise lose entirely, and an automatic waitlist does that without you watching the calendar.
Use the time rather than sitting in it. Photograph your work properly, rewrite the service descriptions you know are vague, tidy the client records, do the training you have been putting off. A quiet fortnight is the only time you will have for the things that make the busy months work.
Be careful with discounting. A price cut fills the diary with people who came for the price and brings your regulars in cheaper than they would have paid anyway. If you need a lever, prefer added value — a treatment included, a longer appointment — or restrict the offer to the specific slots that are actually empty.
And look for the pattern. Most books have a seasonal shape, and a quiet January that arrives every January is a thing to plan around — with packages sold in December, or with the year's holiday taken then — rather than a thing to panic about.