next and the next

Next week, next month etc is the week or month just after this one.
If I am speaking in July, next month is August;
If I am speaking in 1900, next year is 1900.
(Note that prepositions are not used before these time-expressions)

• Goodbye. See you next week.


• I'm spending next Christmas with my family.


• Next year will be difficult.

The next week, the next month etc can mean the period of seven days, thirty days etc starting at the moment of speaking.
On July 15th, 1985, the next month is period from July 15th to August 15th; the next year is the period from July 1985 to July 1986.

• I'm going to be very busy for the next week (the seven days starting today)


• The next year will be difficult. (= the twelve months starting now)