The Nazis invaded on a Sunday. Did this make a difference in the same way when the Japanese attacked Perl Harbor? Did the Soviets have weekends much like the USA? I know back then even in the states a lot people only got Sunday off work. Religion being one of the reasons for that. Since the Soviets had closed a lot of churches and did away with priests and ministers did Sunday make a difference in the attack?
Sunday was a proper holiday in 1941's USSR, in 1940 they've returned to 7-day week from their 6-day week experiment.
And moreover, I think that additional surprise factor was also the time - Germans attacked at night, starting bombing raids about 3.30 am.