Ten-Four Day (4 October)

10-4: Acknowledged; can also be used to denote or emphasize an agreement ("That's a big 10-4.").
"The day was launched by the US telecommunications company Motorola in 1977, when CB radio use was common among police forces, truckers, and amateur radio enthusiasts.
The technology has now been largely replaced by cell phones and other digital devices."
Ah well then...
History, technical stuff, and a list of "10-codes" here:
https://en.wikipedia.org/wiki/Citizens_band_radio
https://en.wikipedia.org/wiki/List_of_CB_slang
[A series of numerical codes for common messages was set up, but when one of these occurred at the beginning of transmission, the equipment of the day often cut off the first syllable. "10-" was added as expendable filler.]