School's off tomorrow, so I'm planning to go out and have fun.
Literal
Tomorrow school [subject-が] day-off [reason-なので] play [purpose-に] go plan-is.
~なので gives a soft reason — 'because school is off.' 遊びに行く予定 combines the purpose expression with 予定 (scheduled plan). The ので causal is softer and more polite than から.