I made an appointment with my boyfriend for 520 days and sent it to tell me how to write it.

You can express it this way: I am very happy during the 520 days with you, every day and every moment. Meeting you in my life is the most precious gift from God. I hope our love will remain sweet in the next 520 days. I wish my beloved a happy life! Or you can express it this way: unconsciously, we have entered the top 520. In these days, I feel your meticulous care and warmth. I hope I can spend the rest of my life with you in the following days.