clarence I thought 'on' is required in the question It is optional. Consider the response: John is available (on) Thursday . The bigger problem is your question form.
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clarenceI thought 'on' is required in the questionIt is optional. Consider the response: John is available (on) Thursday.
clarenceWould it be correct to use "Could I know"?That does not sound natural. This is not an Asian context; it is English, so you can use what I gave you.