On Christmas Day, 1993, Sybill Trelawney, professor of Divination at Hogwarts, joined several of her pupils and fellow teachers for Christmas dinner in the castle's Great Hall. She was fearful upon noticing that twelve people were dining in the hall, believing that were she to sit down, the first of the thirteen seated to rise would be the first to die. Fellow professor Minerva McGonagall expressed doubts regarding her fears.
However, if Peter Pettigrew in his Scabbers disguise is counted as a person, there were thirteen people before Trelawney arrived. Since Dumbledore rose from his seat to greet Professor Trelawney, and assuming Derek and two other unnamed students did not die before Dumbledore's death in 1997, Professor Trelawney's prediction was correct.
In 1995, thirteen members of the order dined together, and the first to rise was Sirius. Sirius was the first to die of those thirteen people.
Thirteen people came to collect Harry Potter from 4 Privet Drive for the last time in 1997. It is possible that this contributed to the bad luck experienced by these wizards and witches on this operation. One of the thirteen, Alastor Moody, died that day.
After Alastor Moody's death, 13 people sat in the living room of the Weasleys. The first person to stand up, Remus Lupin, was the first to die.