Decision made during the day yesterday with 〜うちに

Grammar Explanation: During with 〜うちに

Yukari and her friends have a miscommunication on whether they’ll change from their summer uniforms to their winter uniforms yet. The decision is finalized just in time, as the following day is even colder.

ユカリ:: 「昨日(きのう)のうちに衣替(ころもが)えしておいて良(よ)かった。」; “I'm glad we (took the opportunity and) settled our uniform change (during) yesterday.”; Literal: “I'm glad we did the uniform change within yesterday and left it done.”

Key Points

1昨日のうちに means within yesterday

The uniform decision was made while yesterday was still the available window.
うちに highlights doing something before that time passed.

2しておいて良(よ)かった says the preparation paid off

衣替(ころもが)えしておく means to get the uniform change settled in advance.
良(よ)かった shows relief now that the next day is colder.

3The timing matters more than the duration

This is not about something happening throughout yesterday.
It means the action was completed before yesterday ended.