From Q&A samples
Q: Is the preposition "on" needed in the following sentence? "I will meet with the company president ON April 20."
A: The preposition on before a specific date is usually optional. It is therefore correct to include on in the sentence you mentioned or to omit it. Both versions are correct and sound natural. In certain structures and phrases, English speakers sometimes omit certain prepositions when the meaning of the preposition is clearly understood from the context.