I read that when you refer to a specific date you should use the preposition "on", and if you refer to a month you should use "in". I wonder if this is an unbreakable rule, and, which of the next sentences is more appropriate.

-Individuals were collected on January, 2000.

-Individuals were collected in January, 2000.

-Individuals were collected in January 2000.


On is used for a specific date (e.g. December 20, 2017), because it is one point in time. In is used for a month, because the month contains a time span. So, the described moment is in the span of one month.

So, to answer your question about which is appropriate. Number 2 and 3 are fine. Personally I would say:

Individuals were collected in January of 2000.

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