Let $A$ be the anniversary coin. Select random 6 coins and denote them by set $S$. First, set $A$ and $S$ on the left side and randomly choose 7 coins on the right side of the scale. <1. Equal> Let $B, C$ be the coin that is not used on previous weighing. From the 'Equal' of the first weighing, $B$ and $C$ have same weight. Secondly, weigh $A$ and $B$. If not equal, we know the weight of $A$ and $B$. If equal, $A, B, C$ have same weight. From two 'Eqaul' from first and second weighing, we can know that $S$ is consists of 4 coins different from $A$ and 2 coins same as $A$. Finally, weight $A, B, C$ and random 3 coins from $S$. Then $S$ must contains coin different from $A$ and the weighing result is equal to weighing result of $A$ and the other type coin. So we get the weight of $A$. <2. Not Equal> Secondly, Choose coin $B$ in $S$ and weight it with $A$. Again, we only care when the result is 'Equal'. Finally, Choose coin $C, D$ in $S$ and weight it with $A, B$. If not equal, the result is same as $A$'s weight. If equal, not that the left side of first weighing has 4 coins weight same as $A$.(including $A$) Thus, if left side was lighter, $A$ is the light coin and else $A$ is the heavy coin. From 1 and 2, three weighing is enough to know the weight of the anniversary coin.