Decided to update the numbers that I had jotted down before Min Min was officially announced. Honestly, the averages didn't change too terribly much.
Smash Ultimate Amiibo Waves:
Wave 1: December 7, 2018 (second week)
Wave 2: February 15, 2019 (third week) (70 days between)
Wave 3: April 12, 2019 (second week) (56 days between)
Wave 4: July 19, 2019 (third week) (98 days between)
Wave 5: September 20, 2019 (third week) (63 days between)
Wave 6: November 15, 2019 (third week) (56 days between)
Wave 7: January 17, 2020 (third week) (53 days between) ----- 93 days to April 19th
Wave 8: June 12, 2020 (second week) (147 days between)
Wave 9: September 25, 2020 (fourth week) (105 days between)
Average: 72 days
DLC character release dates:
Plant- February 1, 2019 (first week)
Joker- April 17, 2019 (third week) (75 days between)
Hero- July 30, 2019 (fifth week) (74 days between)
Banjo – September 4, 2019 (first week) (66 days between)
Terry- November 6, 2019 (second week) (63 days between)
Byleth – January 28, 2020 (fifth week) (83 days)
Min Min- June 29, 2020 (fifth week) (153 days between)
Average: 73 days
DLC character announcement dates:
Joker – December 6, 2018 (second week)
Hero- June 11, 2019 (third week) (187 days between)
Banjo- June 11, 2019 (third week) (NA)
Terry- September 4, 2019 (first week) (85 days between)
Byleth- January 16, 2020 (third week) (134 days between)
Min Min - June 22, 2020 (fourth week)(158 days between)
Average: 94 days
Days Between Amiibo Wave and Character Release
Plant: 14 days
Joker: 5 days
Hero: 11 days
Banjo: 16 days
Terry: 9 days
Byleth: 11 days
Min Min: 17 days
Average: 12
So, the dates we get from plugging in these averages are:
September 10th
September 13th
September 24th