I completely forgot!
di-men-ti-CA-to — stress the fourth syllable. 'Dimenticato' is a long word; take it syllable by syllable.
Use with close friends when you genuinely forgot about plans. Honesty is appreciated among friends in Italy, though this works better with familiars than with acquaintances.
'Mi sono dimenticato/a' is the passato prossimo of 'dimenticarsi' (to forget). The reflexive form is essential — omitting it would be grammatically wrong. Adding 'completamente' makes it sound more genuine and less dismissive.
Scusa, mi è completamente uscito di testa.
Sorry, it completely slipped my mind.
Idiomatic — 'uscito di testa' means it left your head. Very natural.
Ho avuto una giornata di fuoco e non ci ho pensato.
I had a crazy day and didn't think about it.
'Una giornata di fuoco' (a day of fire) = a hectic day — adds context
Sono uno/a smemorato/a!
I'm so forgetful!
Self-deprecating and charming — diffuses tension with humour
Italians may express frustration dramatically but rarely hold long grudges among friends. A sincere apology plus a swift arrival (or a coffee offer) typically resolves the situation.