、。
She met him the day before — that is, on May 5.
Literal
She [topic-は] that day [genitive-の] previous-day, namely May 5 [time-に] him [to-に] met.
つまり ('that is, in other words, namely') is a useful conjunction for restating or clarifying — exactly what English uses 'i.e.' or 'that is to say' for. Here it bridges the relative time-frame (前日, 'the day before') with the calendar date (５月５日). 会う takes に for the person met (彼に, not 彼を). May 5 is no random date in Japan: it's 端午の節句 ('Boys' Day' / Children's Day), a public holiday during Golden Week, marked by carp streamers (鯉のぼり) and samurai-helmet displays. A learner familiar with the cultural calendar may suspect something is encoded by the choice of date.