Historical clue formulations

• Event first observed in 1970 — 2023-02-16
• Observance first celebrated in 1970 — 2020-05-10
• April 22 — 2020-04-12
• April event that's "green" — 2020-01-14
• *Scarcity — 2019-08-07
• Environmentalists' celebration — 2004-12-20
• April celebration — 1998-01-21