が usage in 昨日が忙しかったです


When translating "I was busy yesterday," why is this incorrect?

> 昨日が忙しかったです

Isn't it the same as

> 私は昨日が忙しかったです




Because the subject is changed from 'I' to 'Yesterday'. Please translate it
into 「私は昨日忙しかったです」. Omitting the subject,「昨日は忙しかったです」is more natural in daily
conversations. But 「昨日が忙しかったです」 is an expression to identify the day you were
busy. So it is correct for the answer to the question 'Which day was a busy
day for you?'

