Dear Sarah,
I am so incredibly sorry that I missed your housewarming party last Saturday. I was really looking forward to seeing your new place and catching up with everyone, so it was a huge disappointment to realise I had let you down.
To be honest, I completely mixed up the dates. I had marked the event in my calendar for the following weekend, and it wasn't until I saw the photos you posted on social media that I realised my mistake. I felt absolutely terrible when I saw how much fun everyone was having and realised I had missed the celebration entirely. Please accept my sincerest apologies for my absent-mindedness.
I would love to make it up to you soon. Are you free next Friday evening? I would like to take you out for dinner at that new Italian restaurant in the city centre to celebrate properly. Alternatively, if you prefer, I could pop over with a bottle of wine and some dessert later this week to see the flat properly. Let me know what works best for you.
Best wishes,
James