In The Graveyard Book, why did mistress Owens push Bod out into the cold?

Bod's adoptive mother Mistress Owens makes him go outside to play. Bod's none too thrilled at this because it's absolutely freezing cold. But Mistress Owens insists; she wants Bod out of the way so she can tidy up the tomb in preparation for the Macabray, a weird ritual in which the living and the dead join together in a dance.
The Macabray is clearly a big deal in the graveyard as all the spirits are busily going about cleaning and dusting. In fact, they're so busy that they have no time for Bod, which is unusual. However, when the Macabray finally gets under way, Bod enters into the spirit of things, dancing away with the Lady on the Grey, which he finds to be a most enjoyable experience.