The schematic for this is simple. A power plug leads to a master on/off switch. The output of the master switch feeds the first switch in each month column. Each switch in a column is wired in series to the next. The last switch leads to a 1/8W resistor and an LED. The other leg of all the LEDS are tied together and return down the left side of the board to the power plug.
The switches were selected because that's what I was using for the Sinclair Scientific Calculator project and was familiar with them. The center pin is the common. If the stem is thrown to the left, the center and left pins are bridged. If the stem is thrown to the right, the center and right pins are bridged.
You can feed the switches from the left pin and get the output from the center or feed them from the center and get the output from the left pin.
The polarity of the power plug is not marked. The supply voltage is not marked. The size of the resistor is not marked. The LED polarity is a suggestion. All of those details are left to the user. If in doubt, solder the master switch and a switch column first and then work out the details. I ended up selecting a 9V plug, 4.7K resistor and a blue led, because those are the things I had on hand.
The next step is to design a laser cut base and alignment jig to grab all the switch stems.