What's the difference between "in January next year" and "in January of next year"? In what instances do we need to use "of"? Thank you.
Both forms can be used.
The form without 'of' is less formal and is normally used in conversation.
The form with 'of' is more formal and would be more appropriate for written texts.
Note that you have to use 'of' when you for instance say or write 'the end of next year' or 'the middle of next year'.