Look at how we say next and last with durations:
In French when you say next/last + [generic duration term] (month, week, year...), you will use :
le/la/l' + [duration term] + prochain/prochaine (next)
le/la/l' + [duration term] + dernier/dernière (last)
Note that to say the next day or the following day in French, you use le lendemain, and the previous day/the day before is la veille.
See also the more advanced When to use "demain"/"hier" vs "le lendemain"/"la veille" vs "le jour suivant"/"le jour précédent"
ATTENTION:
For specific days (next Monday, last Friday, ...), you won't use the definite article le :
[day of the week] + dernier / prochain
For specific months (last February, next March, ...), you will use :
en + [month] + dernier / prochain
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