Had yet another bad case of morbid curiosity so I decided to look into the history of major firmware updates with the Switch and these were my findings:
en-americas-support.nintendo.com
Firmware 3.0 - week after E3 2017
Firmware 4.0 - week before Pocket Camp Direct
Firmware 5.0 - half week after Spring 2018 Direct
Firmware 6.0 - half week after Fall 2018 Direct
Firmware 7.0- two weeks before Spring 2019 Direct
Firmware 8.0- day before Joker release
Firmware 9.0 - half week after Fall 2019 Direct
Firmware 10.0- three weeks after March Mini 2020 (half week before Min Min's Amiibo wave original release)