Dear Mrs Green,
I am writing to apologise for the noise coming from my flat last Saturday night. I hope it did not disturb you too much, and I am truly sorry if it did.
I had invited a few friends to celebrate my birthday, and I was expecting a quiet evening with about eight people. In the end, some others arrived unexpectedly, and the music got louder than I had intended. I did not realise how late it had become, and I only turned it down when somebody came to the door at about one in the morning.
I understand that you have to get up early for work, and I feel very bad about spoiling your night's sleep. It was thoughtless of me, and I should have warned you beforehand.
Next time I plan a party, I will let you know in advance. I will also make sure that the music is switched off by 11 pm and that guests leave the building quietly. If you like, I could also invite you to join us.
Once again, please accept my apologies.
Kind regards,
Jakub Zielinski