、。
If it rains tomorrow, I'll stay home all day.
Literal
Tomorrow rain [subject-が] fall-if, all-day house [at-に] exist.
一日中 ('all day long') is a time-span adverb using the suffix ~中. Combined with a ~ば conditional, the sentence makes a routine plan: if rain comes, the day is spent indoors. ~ば conditionals attach to the verb stem and pair naturally with this kind of habitual or projected outcome.