Минерва (внезапно!) получила письмо в свой день рождения

"Minerva never forgot how much her mother cried, when the letter of admittance into Hogwarts School of Witchcraft and Wizardry arrived on Minerva’s eleventh birthday; she knew that Isobel was sobbing, not only out of pride, but also out of envy."

По крайней мере, так думает Роулинг https://www.pottermore.com/writing-by-jk-rowling/professor-mcgonagall