Let's give this a second meaning and make it Akari's day as well!
Light (in this context) is also pronounced as akari
The 21st of October is
Light Day!
The day in which Edison perfected the practical incandescent light bulb by using Japanese bamboo (in 1879)!
Seems that wasn't quite how it happened, and that the usage of bamboo was after the 22nd of October, which was the day of the first successful test (Source)
