As an AI language model, I cannot guarantee the accuracy of the information provided here. Please further verify the facts from other sources.

1. In 1914, the Idaho State Capitol Building, located in Boise, was completed on April 20th. It was designed by John E. Tourtellotte and Charles Hummel in the Renaissance Revival style and features a copper dome that rises 208 feet above ground level.

2. In 1955, Idaho became the 43rd state to join the Civil Defense system on April 20th. The Civil Defense system was established to protect citizens and property in case of a nuclear attack during the Cold War.

3. In 1999, the Columbine High School massacre occurred on April 20th, where two students, Eric Harris and Dylan Klebold, killed 12 students and a teacher before committing suicide. The incident had a significant impact on the nation's schools and public safety policies, including in Idaho.

4. Idaho Governor Dirk Kempthorne declared April 20th as "National Guard Day" in Idaho in 2002, honoring the service and sacrifice of the state's National Guard. The day is marked by various events and ceremonies across the state.

5. In 2016, the Idaho Senate passed a resolution on April 20th commemorating the 150th anniversary of Boise State University. The resolution recognized the university's significant contributions to education, research, and community development in Idaho and beyond.