。
Yumi is leaving for Osaka the day after tomorrow.
Literal
Yumi [topic-は] day-after-tomorrow Osaka [to-に] depart.
旅立つ ('to depart on a trip, set out on a journey') is a slightly literary compound of 旅 (journey) + 立つ (stand, set out) — more evocative than plain 行く. あさって ('the day after tomorrow') is a relative-time adverbial, no particle needed. 大阪 is, of course, Osaka — Japan's second-largest metro area.