I understand "next season" to be an idiom which functions as an adverb. ) In this particular example, " the next season" cannot be substituted. At the beginning of a sentence, it seems to be optional: [The] next season, they did better.
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AnonymousMy question is, why there is no article before "next season"?Because, as Avangi said, it's an adverbial expression used with present time.