Look at these straightforward examples of où as a relative pronoun:
La maison où j'ai grandi a été démolie.The house where I grew up has been demolished.
La ville où il habite a beaucoup de charme.The town where he lives is very charming.
Here où simply means where and introduces an action taking place in a previously mentioned location.
See also Dans lequel / laquelle : alternative to où with places (relative pronouns)
Now look at these sentences using où in a different context:
Le jour où il a gagné le match.The day (when/that) he won the game.
La semaine où ils sont restés à l’hôtel s’est vite terminée.The week (in which) they stayed at the hotel went quickly.
L’époque où il vivait était dangereuse.The era (in which) he lived was dangerous.
The era he lived in was dangerous.
In sentences where the relative pronoun introduces actions taking place during a previously mentioned time frame, you will also use où to express when or that/in which.
This can lead to completely different ways to express things than in English:
Le jour où j'ai eu vingt ans...The day (when/that) I turned twenty...
Note that you cannot omit the où the way you can the when/that/which in English.
You can never use quand or que in this context!
When used in a question, où always means where? and quand always means when?
Où vas-tu ce weekend ?Where are you going this weekend?
Quand vous êtes-vous rencontrés ?When did you meet?
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