The reasons for the postponement of Barbarossa from the initially planned date of 15 May to the actual invasion date of 22 June 1941 (a 38-day delay) are debated. The reason most commonly cited is the unforeseen contingency of invading Yugoslavia and Greece on 6 April 1941 until June 1941.[112] Historian Thomas B. Buell indicates that Finland and Romania, which weren't involved in initial German planning, needed additional time to prepare to participate in the invasion. Buell adds that an unusually wet winter kept rivers at full flood until late spring.[113][k] The floods may have discouraged an earlier attack, even if they occurred before the end of the Balkans Campaign.[115][l]
OKH commander, Field Marshal Walther von Brauchitsch, and Hitler study maps during the early days of Hitler's Soviet campaign
https://en.wikipedia.org/wiki/Operation_Barbarossa#German_preparations

[This isn't too bad but I don't rate Shirer or Beevor]

The importance of the delay is still debated. William Shirer argued that Hitler's Balkan Campaign had delayed the commencement of Barbarossa by several weeks and thereby jeopardised it.[117] Many later historians argue that the 22 June start date was sufficient for the German offensive to reach Moscow by September.[118][119][120][121] Antony Beevor wrote in 2012 about the delay caused by German attacks in the Balkans that "most [historians] accept that it made little difference" to the eventual outcome of Barbarossa.[122]