Could you guys tell me if there is any difference between the following:
Il est tard. On y va!
C'est tard. On y va!
If so, does it apply also to:
Il est lundi (aujourd'hui).
C'est lundi (aujourd'hui).
On est lundi (aujourd'hui).
Time of the day and days of the week do not work the same way:
Time of the day:
Days of the week:
In usual -but not bad- language,you could use both, but in order to be precise it depends if you have knowledge of the time when you say the sentence.
You have time on mind: Il est tard on y va. is a more pragmatic sentence.
You take knowledge of the time at the moment: C'est tard on y va. could be said.
Something is pointed by the "C'" (an object) on this case this is the time. The correct but a bit more valuable sentence in all case is more Il est tard But in order to be more pleasant like on a night event ending: C'est tard on y va is more appropriate as I use it.
Il est lundi (aujourd'hui). C'est lundi (aujourd'hui). On est lundi (aujourd'hui).
Are really the same, without any language level sort.
As i see it:
-C'est lundi : More used in the north, it depends.
-On est lundi Probably the best to say in the south where the singing accent can be heard.
Il est tard, C'est tard, even Il se fait tard, are all synonyms, and the only difference is the language level (Il se fait tard being the most formal one).
As for your second question, both C'est lundi and On est lundi are correct. Il est lundi is not.