かつて + [sentence] かつての + [noun] かつてない + [noun] かつて + [verb た-form] + ことがない
かつて is an adverb meaning "once", "at one time": something was so a long time ago and is not so now. It goes at the head of the clause it belongs to, and the verb is in the past.
Before a noun it takes の and means "former": かつてのゆうじん友人, a former friend.
With a negative, かつて means "never before": nothing like this has happened up to now. かつてない before a noun is a set phrase, "unprecedented".
かつて is written Japanese: history, news, speeches. It also reaches far back, to another era, so it does not suit last month or last year. In conversation you say むかし昔 or まえ前は.