Why is October 1 the feast day of St. Therese of Lisieux?

There is no firm rule that says a feast day must be on the anniversary of the death date of a saint. Any date can be chosen and the date need not correspond to any event in the life of the person. Therese died on September 30 in the evening so her feast day is on the day after her death. Liturgically and historically speaking, the new day begins the evening of the day before. Therefore, Therese would have technically died on October 1.