It was never so cold in the second half of April.
According to the weather service MeteoNews, the weather in the second half of April has never been so cold in at least six places, at least since the start of the measurements. Among them are Visp in the canton of Valais, where it has been measured since 1959, and Arosa in the canton of Graubünden, which has been measured since 1931.
The temperatures were clearly below the freezing point on Thursday night, in Visp about minus 5.5 degrees and in Arosa about minus 12.8 degrees.
Thanks to H.B. Schmidt for this link