For David's 41 block, I was stumped. All I could think was that it's a prime number, so I did a little googling to see what else I could come up with.
Turns out 41 is also a centered square number, which means "the sum of two consecutive square numbers and are congruent to 1 (mod 4)." Yeah, mathy, but the way it is graphed out is beautiful. The dots in the square keep radiating out (there are only 13 here, the next round would be 25, then 41, and so on), which is kind of like a birthday--with each round, or year, you gain so many more dots of experience. Then how the number is a sum of two consecutive square numbers--so 41 is the sum of 16 and 25--is also a nice way to think of birthdays. You reflect on the past, on your 16th and 25th birthdays and the years that followed, and how added together, that's how you got to where you are now, at 41.
A lot of math has gotten you to this age, but it's not without even more nuance and art. Happy birthday, David, and here's to many more centered square ones!