The Gregorian calendar had been used by most of the world dating back to its introduction by Pope Gregory XIII back in 1582. Since the modern age, most of us buy a yearly calendar (or use our phones), while others save their money by using a perpetual calendar, which can be difficult to interpret using complicated tables if they are non-digital.
To overcome that issue, Andrei Erdei designed his Backlight Automated Perpetual Calendar, which makes it easy to read the date using illuminated numbers. “To display the days of the current month," Erdei explains, "I use a strip of LEDs placed behind a paper screen, on which are printed the numbers of the days of the month, only the necessary LEDs light up to show the days of the actual month."
Erdei created his calendar using CNC-machined acrylic smoked panels sandwiched together with a series of WS2812B LED strips embedded between them. The LEDs are driven by an ESP-01 (ESP8266) module, which grabs the current date from an NTP server, and illuminates the corresponding numbers (for weekdays and weekends) on the calendar. The calendar face was made using a stencil and then printed on a piece of paper with an opaque background, which was achieved by printing several times over on the same sheet of paper.
Although Erdei used a CNC machine to cut his acrylic panels, he states it can also be done using a laser cutter or even a 3D printer, as long as it can fit 235 x 120mm parts. Erdei has uploaded a complete walkthrough of his Backlight Automated Perpetual Calendar on his project page for those who would like to recreate his build.