"It is a day."
Translation:Es ist ein Tag.
I think you mean "Feierabend" :)
Well..... "Stop complaining about having no internet! It'll be back tomorrow! Es ist ein Tag! You can go without internet for one day!" (But you wouldn't say "It is a day" for that, of course.)
Same as in English, the sentence seems uncommon so far; you could add e.g. "It is a day on which children hang their socks on the mantelpiece and hope somebody will fill it with presents"...