Is 'on' necessary?

[kohyoongliat]

I will be available in the office (on) Wednesdays and Thursdays.

Is 'on' optional? Or it is better to have it in the sentence? Is it a case of British vs American English?

[Mister Micawber]

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My answer remains HERE.
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[kohyoongliat]

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My answer remains HERE.
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I believe 'on' is needed in BrE, but usually left out in AmE. Correct me if I'm wrong.

[udara sankalpa]

Hi
I would say that it's a matter of being formal & informal in using English Language. That is, In our day to day speach we can drop "on", or else it has to be there, gramatically speaking.

Cheers
Udara