Four referendums were held in Switzerland in 1991. The first two were held on 3 March on lowering the voting age to 18, which was approved, and on a popular initiative on promoting public transport, which was rejected. The third and fourth were held on 2 June on reorganising the federal finances, which was approved, and amending the military penal code, which was approved.
References
Swiss referendums, 1991 Wikipedia(Text) CC BY-SA