Question: when was the actor revealed as "The Mother" in the Season 8 finale in  How I Met Your Mother born 
Sentence (from "How I Met Your Mother (season 9)"): Cristin Milioti, who was revealed as "The Mother" in the Season 8 finale, was promoted to a series regular, the only time "How I Met Your Mother" added a new regular cast member.
Relevant evidence: Yes