Double Diamond Calendar Puzzle, Triangular Grid

Introduction

Calendar puzzles can be found in all sorts of shapes, not just squares! This post is about a fun one using a triangular grid. You can see the board layout and pieces below.

The board on the left is shaped like a diamond, made up of 56 equilateral triangles. It’s divided into three sections: the upper left for months, the lower one for days of the week, and the rest for days of the month. Each equilateral triangle has a date element, except for six that are empty. Most of these empty triangles are needed to make the puzzles solvable.

Now, the nine pieces above are also made of equilateral triangles. These shapes are called polyiamonds, and here we use one heptiamond, six hexiamonds, and two pentiamonds. and here we’re using one heptiamond, six hexiamonds, and two pentiamonds. You might have noticed one of the pieces is a diamond! That’s why this puzzle is called Double Diamond because of the small diamond piece and the large diamond board.

You can find the svg file for laser cutting/engraving machine at CuttingPath and ATOMM.

How to Play?

The goal of this puzzle is to cover all the triangles, including the empty ones, with the nine polyiamond pieces, except for the target date.  An example solution for Jan 1st Monday is shown below. You can pick today or any date of interest, of course. There are about 69 solutions per date on average from all 2,562 (=366x7) dates, and every date has at least 3 solutions. However, the triangle-based puzzle can be more challenging than the square-based one, especially if you’re not familiar with the 6-fold rotation every 60° with polyiamonds.

If you give this a shot, I’d love to hear your thoughts in the comments below!