The Castle In The Mist by Amy Ephron

Tess and Max are brother and sister. As with most siblings, they fight, one of these fights leads to Tess finding a gate and a key. The gate has no fence. At least not one that she can see. She opens the gate with the key and meets William, a strange boy who lives with a governess and a groundskeeper. He gives Tess a mysterious warning: Beware the hawthorn trees. But what does that mean? Tess comes back the next day with Max. This is when they are invited to dinner with William. When they go to dinner, it’s a blue moon, and a blood moon. Horses, magic, danger. Tess encounters all these things when she must save William and her brother. Enjoy reading it, you’ll have a hard time putting it down.