This is a very basic principle: you use "in" for periods of time. "On" is used for days and dates. "At" is used for specific points in time which are met (eg. hours, but also for anticipated dates).
"In early January." is the only one which makes sense.
2011년 12월 21일
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"In" :)
2011년 12월 21일
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Thank you!
2011년 12월 21일
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bbb eki is right " in early January" is the correct one to use in a sentence