The question "when do they find out Mona is a" is incomplete, but based on the search results and common interpretations, it seems to refer to when people find out Mona's identity as "A" in the TV series Pretty Little Liars. In Pretty Little Liars, Mona Vanderwaal is revealed to be the original "A" (the mysterious antagonist) in the mid-season finale of Season 2, specifically in the episode titled "Unmasked." This discovery is crucial in the storyline as it unravels part of the mystery surrounding the anonymous tormentor of the main characters. Mona reveals to Spencer and the others that she was "A," driven by her feelings of betrayal and her complex relationship with the protagonists. This revelation happens in season 2 when Mona's identity as "A" is disclosed to the Liars and the audience.
If the query was meant for a different Mona or context, please specify for a more precise answer.
