When?

For 1940-43 the Germans were able to make daylight raids prohibitively costly for the British, who switched to night raids as a result. In 1943 the Americans tried their hand at daylight bombing and suffered unsustainable losses. After a pause and the introduction of long range escort fighters the Americans resumed their daylight campaign in 1944, forcing the Luftwaffe to defend German airspace. The result was a war of attrition that the Luftwaffe lost over a period of months. The Luftwaffe was not able to mount a defense on D-Day and rapidly lost control of the skies over Germany for the rest of the war (with a couple raids being the exception).

This video was shown to incoming American bomber crews to show them why they used certain tactics against German anti aircraft artillery, but it can be watched to learn how the Germans employed their AAA/flak as well.

Is there a specific time you meant? Or is there an aspect of how air defense was used that you have a question about?