The day after tomorrow, I'm planning to go to the ballpark to watch baseball.

Literal

Day-after-tomorrow baseball-field [to-に] baseball [object-を] see [purpose-に] go schedule [copula-だ].

明後日 (あさって) = 'day after tomorrow.' 見に行く is another purpose に + motion pattern — 'go to see.' 予定だ rounds it off as a scheduled plan.