Ochi Day.
The day the Greeks said No to the passage of Italian forces into Greece, Italy backed by Hitler wanted free passage into Greece but the refusal by General Ioannis Metaxas' to allow this brought the Greece into the war, and it was Greek forces who pushed Mussolini's forces back trough Albania.
Some historians believe that if the Greece had agreed to the request that Hitler would have Invaded Russia in the spring,instead of making his disastrous attempt to invade during the winter.