In general, these relative time phrases with 'next' don't take 'the'. 'Next week', 'next year,', next May', 'next time' - in the same way as 'last' ('last week', 'last Monday', etc).
But they do need the article when you're talking about something specific.
'The next time I see you, I'll be married.'
'The next slot the surgery has available is on Monday.'
'The next week you have off is in June.'
'The last time she saw him, he was looking ill.'
In your example, both are possible depending on context. If you say 'the next week', it's likely you've been looking at a succession of weeks, for instance. But if it's just a general relative expression of time, 'next week'.