What date on resignation?

[Prettyprettygood]

I’m writing my resignation letter, with 4 weeks notice my last day will be 23rd of December but on the letter do I write resigning effective from the beginning of notice period or end of notice period?

Keep it simple. Don't say "with effect from" or similar. Say something like "Please accept this letter as my resignation. As I have to give 4 weeks notice my final day will be 23rd December".