what does this mean - why do you say atsukatta - was hot - but finish the sentence with kinou desu which is present form

ichiban atsukatta hi wa kinou desu

Because 'desu' is a copula that goes at the end of the sentence, it acts like an auxiliary verb. Because it acts as an auxiliary verb, forming the past tense can be done in 2 ways: 1.) Main verb in past form + present form of desu -> Atsukatta desu = it was hot 2.) Main verb in present form + past tense form of desu -> Atsui deshita = it was hot. Either way is correct and the sentences both mean the same thing. Putting both the main verb and 'desu' in the past tense is gramatically incorrect.