I arrived the day he left?

Question

I believe that “I arrived the day he left” refers to a non-specific timeframe and therefore should use the feminine form, but it was marked wrong on my test and the masculine “la jour” was said to be correct instead. Can someone explain to me why this would be the case, or if it’s an error?

Accepted Answer

This is definitely a difficult concept for non-native French speakers.
"Le jour" is correct as this is 'the day he left' - a specific 'point/period' in time, rather than referring to a duration. ("La jour" as you typed in your question is not correct, nor would "la journée" have been). 
Maybe an example of both uses in the same sentence may help: 
"Il travaillait toute la journée le jour où je suis arrivé" - 'he was working all day (on) the day that I arrived'.
In the first clause 'day' is used as a duration, in the 2nd, it is used as a 'time point'.