I hadn't factored in that it was today.
'Messo in conto' — MES-so in CON-to. Both nouns carry stress. The phrase is idiomatic.
Use when you made plans but then forgot to account for them when making other arrangements — a more sophisticated expression of having double-booked or forgotten.
'Mettere in conto' means 'to factor in' or 'to account for'. 'Non avevo messo in conto' is the piuccheperfetto (past perfect) — 'I hadn't accounted for'. Using this tense shows B2-level grammar and sounds mature and thoughtful.
Mi sono sovrapposti gli impegni.
My commitments have overlapped.
Very formal — 'sovrapposti' (overlapped) sounds very organised and professional
Ho fatto un casino con le date.
I messed up the dates.
'Casino' here means mess (not a gambling house) — very informal and honest
Ho prenotato qualcosa senza ricordarmi di questo.
I booked something without remembering this.
Explains exactly what happened — slightly formal, very clear
At B2 level, Italians appreciate nuanced apologies that show you understand exactly what went wrong. A vague excuse can feel dismissive; a specific one shows respect for the relationship.