For German daily data, start here at the Deutschewettersdienst website.
Open the file KL_Tageswerte_Beschreibung_Stationen.txt to find a station with data for the location and time period that you want. For Bonn, it looks like station ID 599 "Bonn-Friesdorf" covers 1932 through 1999; for Cologne, station ID 2665 Koln-Botanischer Garten covers 1936 thru 1984.
Then, find the zipfile for that station, e.g. tageswerte_00599_19321001_19990301_hist.zip
Within the zipfile will be a large textfile named something like produkt_klima_Tageswerte_19321001_19990301_00599.txt which contains daily records of temperature (avg, high, low, air pressure, precipitation, etc.) It's raw data, so you'll have to find the column headings at the top, then read down the rows until you find the date that you want.
The only problem is that, for fairly obvious reasons, the stations stop recording and have data gaps beginning in 1944 or 1945, resuming again in late 1945 and 1946. However, I have found two Berlin stations - 417 Berlin-Lichterfelde(Sud) and 402 Berlin-Dahlem (LFAG) - that continue recording up to March 31, 1945 and April 24, 1945, respectively.
Makes you think.