I know, for a specific date, it is used on: On 16 May 2016.
I also know, when you refer to a month or year alone you use in: In June, In 2000
But I saw in internet some sources that when you put together a month and the year, they use both "on" and "in". I suppose that the correct form is In May of 2000.
Which form is correct? On May of 2000 or in May of 2000?
Also, is it necessary to put "of" or not? For example: May 2000 or May of 2000