Try not to cheer too loudly as you read this letter, for the day you have been waiting for has arrived. I'm resigning!
I finally (graduated/won the lottery) and will be moving on with only the fondest feelings for my time with (the company). My last day will be (date).
I will be bringing cupcakes to the office on my final day, and I hope everyone will join me in celebration. If cupcakes aren't your thing, feel free to buy me a good-bye drink after work.
I am truly grateful for everything I have learned here, and wish the company and my colleagues all the best.
Oh, and yes, I was the one who never refilled the ice cube trays in the office fridge. Sorry!