"I mean ... it's all looking serious, isn't it?"
-- Hermione, discussing the mounting danger the day after (HBP11)
Hannah’s mother was found dead, killed by Death Eaters. Hannah is called out of Herbology class to receive the horrible news and leaves school for awhile.
This happens the day before Harry is to hold Quidditch try-outs, which is the second Saturday of the fall term. On the real world calendar, this would be Friday the 13th. The date of Mrs Abbot's death might be a day or two earlier, since the 13th is when they actually found her body (and when Hannah is removed from class, since it is unlikely that they would have waited to the next day to inform her).
Although it is not stated, Hannah's mother was very likely Muggle-born, which might have been why she was targeted by the Death Eaters.