This question already has an answer here:
When talking about months should I use the preposition "on" or "in"?
For instance,
I am going to be there on/in April.
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marked as duplicate by choster, Nathan Tuggy, DJMcMayhem, M.A.R. ಠ_ಠ, StoneyB Mar 29 '16 at 11:17
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2"in April" but "on April 15th" – user3169 Mar 29 '16 at 0:17
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To expand one of the comments, you would use "in April" because the day when you will arrive is within the month of April, but if you say a specific day, you would say "on April 15th". The key is that using "in" means what you are referring to encompasses something else, while on usually refers to a specific day.
You would say that you wake up "in the morning" rather than "on the morning" since it is a more general time. You would use "at" to describe an exact time, such as "at noon", or "at 5 o'clock" rather than "in 5 o'clock" or "in noon" or "on noon".
Here is a helpful reference.
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1It would be better to close the question in favor of the more complete duplicate than to give an incomplete answer here. For instance, you would say you woke up at night, but not at morning, and while you wouldn't say on morning, you would say on Tuesday morning. Depending on your dialect, you would say on the weekend or at the weekend; likewise, seeing my mother at Easter is somewhat different on Easter. And while we use in for most periods, like in the reign of Tiberius or in the rainy season or in the future, it doesn't work with in the holidays for example. – choster Mar 29 '16 at 2:53
