I have read various opinions on this subject, and my English professor told me that:
When the definite article is used, it indicates the period itself, like:
I will do it in the next week
= in the seven days from now (might be from Tuesday till Tuesday).
Next year - 2019. In the next year: in 12 months from now, so in April 2019.
I will meet you next week. //next week, might be tomorrow if today is Saturday
I will meet you in the next week. //in the next 7 days
Is that correct?