Let me try to clarify:
We meet Mondays. -- On se rencontre le lundi.
In examples like this, where you would use the weekday in its plural form in English, you simply use the definite article and singular in French.
However, if you wanted to say:
We meet every Monday. -- On se rencontre tous les lundis. Or: ....chaque lundi.
So, apparently, the plural version of weekdays does exist but you use it differently than in English. When you would say "every Monday" in English, you can say "all Mondays" in French and use the plural.
-- Chris (not a native speaker).
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