Breakdown of C'est une journée sensationnelle !
être
to be
une
a / an (feminine)
ce
it / this (with être)
la journée
the day
Questions & Answers about C'est une journée sensationnelle !
Why do we use journée instead of jour in this sentence?
Would it be wrong to say C'est un jour sensationnel?
Why do we start the sentence with C'est rather than Il est or Elle est?
You must use c'est because it is followed by a modified noun phrase (a determiner + noun + adjective: une journée sensationnelle). You would only use il est or elle est if it were followed by an adjective alone (for example, if you were already talking about the day, you could say Elle est sensationnelle !).
Why is there a space between sensationnelle and the exclamation mark?