"What are you doing tomorrow?" is the common Engpsh way to ask someone about their plans for the next day.
Source: This is a commonly used Engpsh phrase, originating from the Engpsh language and culture.
Examples:
"What are you doing tomorrow?"
"I was wondering what your plans are for tomorrow?"
Translations:
"明天你要干什么？"
"我想知道你明天的计划是什么？"