She doesn't know when this eight p.m. appointment will be over, but she needs to be back in her own house by midnight. She can't risk them catching her in the house of the Weasleys. If the Church of Hogwarts is a front for the demon-hunting Order of the Phoenix, and Ginny is attached to Harry who's attached to the church, it's a fair guess that she's mixed up with the Order as well. Draco definitely does not need to find out about Hermione's actions today.