I just played a game called Company of Heroes which shows the beginning of the D-Day invasion from the German perspective. The German soldiers are just lounging on their AA guns joking about the coffee tasting like shit when they start to see tracers in the distance. They all scramble to turn on the spotlights and the sky is filled with C-47s and gliders. This had me daydreaming about how absolutely terrifying D-Day must've been to the Germans seeing the combined might of the allied forces descending upon them.
So how much of a heads up were they actually given? Did they have some sort of time frame? Were they caught completely unaware?
Thanks.
They honestly were not given that much of a warning. Germany knew eventually an invasion would come but didn't know exactly when or where. Operation Fortitude was a deception operation to make the Germans think the invasion forces was landing in a different area and not the Normandy beaches. This operation was fairly successful in pulling attention away from Normandy. But Germany's Atlantic Wall still made sure an invasion anywhere was going to be meet with heavy resistance. Now I highly doubt any Germans were just lounging around as if nothing were to happen. The Germans did have an idea that the allies were planing an invasion just not where it was to land.
several times operation overlord almost got exposed by German intelligence but never to the point the operation was uncovered.
Long answer short: They didn't when they should. Only the 15Th army (roughly 200km North of Normandy) was alarmed of a possible invasion in the next couple days by a message, the 7Th (who guarded the Normandy coast) was not alarmed of this message, Rommel went to Germany for his wife birthday, the higher generals and commanding officers left the front to train for and simulate a possible Normandy invasion (the irony), the Luftwaffe 26th Fighter Wing was moved from the coast and only 2 fighters stayed and when the Invasion did happen, the German command didn't believe it was the real invasion and many of the soldiers first thought it was false alarm.
Long answer, used The Longest Day by Cornelius Ryan as source. I've the Dutch copy, so quotes are translated to English from Dutch and not a 1:1 copy. I will mainly go about the first week of June and not the operations to put the German forces off, like Operation Fortitude.
Rommel
Rommel wrote to his wife on the 6th of May that "Their still is no sign of the English and the Americans... Every day, every week.. we grow stronger. I look with confidence towards the battle... Maybe it will start the 15th of May, or maybe at the end of the month". However, in his rapport to the OB West (Oberbefehlshaber West, command of the Western front) he stated that there was a increase in messages to the France resistance and that the Allies had made preparations for a invasion. "But, from pas experiences we known that this doesn't necessarily mean that an invasion is about to happen".
On the 4th of June he left the front to be with his wife on her birthday, the 6th of June. This, without the HQ knowing. Only General-Mayor Schumndt knew about Rommels trip. The others at the OKW thought that Rommel was at the front when the invasion happened.
Messages
In the morning of the 4th of June at the 15Th Army headquarters, more then 200KM north of the Normandy beaches, Lieutenant-Colonel Meyer was tasked with the command of the only counter-espionage service at the invasion front. This group of 30 men did nothing else then listening to radio messages from across the channel. In the night before, the next message was received on the press telegram:
"URGENT - PRESS ASSOCIATED NYK FLASH - EISENHOWER HQ ANNOUNCES ALLIED LANDINGS IN FRANCE".
First Meyer waited before alarming Berlin. First reason for this was that there wasn't any Allied activity across the entire front. Secondly, Canaris, head of the German Secret Service, told Meyer that there where going to be hundreds of false messages to the France resistance and only a couple would be relevant to the D-day Invasion. He concluded that the message would be a mistake, since there wasn't an invasion going on.
A much more important message was received the 1st of June. "Les sanglots des violons de l'automne". This is the first sentence of the poem Chanson d'automne. Canaris had informed Meyer that this would be the first part of a two part message. The first one would be on the 1th of 15th of the month and announce that the invasion would be in the coming 2 weeks. The second part (the rest of the poem) would announce the invasion being within the next 48 hours.
Meyer informed the staff sergeant of the 15Th army of the message who immediately gave the order the put the entire 1th army on full alert. The message was sent to OKW (Hitlers HQ) and Meyer called OB West, Army group B and Von Rundstedt HQ. And that was the end of that... OKW assumed that Von Rundstedt already had sounded the alarm, and Von Rundstedt assumed that Rommel's HQ already sounded the alarm. This caused that on the entire invasion coast only 1 army was on full alert. The 7th, who was tasked with the Normandy coast never knew.
On the 2th and 3th of June the first part was repeated. That convinced Meyer that the AP message must have been a mistake. The invasion was about to happen, and his man would be ready as never before. He only hoped that his superiors would be in the same state as him. Meanwhile, Rommel prepared his trip to Germany.
Re-stationing of the 26th Fighter Wing
There where only 183 day fighter planes in France, of which 160 considered serviceable, of which 124 belong to the 26th Fighter Wing. In the afternoon of the 4th of June, 122 of those fighters where re-stationed elsewhere and moved back from the coast (a squadron to the North-East of France, a squadron to the Rheims and the 3th squadron to the south of France).
This left the coast almost completely without any airpower. Only Colonel Priller, one of Germans top aces, and his wingman stayed behind. Even though they where only with 2, they did saw action during the day. They made a couple low passes over the beach and shot at the Allied soldiers who where trying to get of the beach, after which they flew off.
Weather and false alarm These subject are more widely known and I will skip a bit through it. In the past the Allied had shown that they wouldn't to airborne landings in bad weather. Guess what, it was bad weather. The Germans where convinced that the invasion wouldn't happen with such bad weather.
When the first troops landed, people thought it was just false alarm. And when the first fights broke out, people thought this wasn't the invasion and only a diversion.
Round-up
The Germans made mistake after mistake after mistake. They had reason to think the invasion would happen in the next couple of days, but thanks to bad communication the message never came through. When it did happen, the commanders that weren't at the front didn't believe it was the real invasion.
To put it in perspective. Here's a conversation over the phone between Pluskat, who was in one of the bunkers over looking the Omaha beach front, and Block, mayor of the 352th Division:
I can't describe the chaos in the first 9 hours of D-day. I really recommend reading The Longest Day. It very easy to get into and it reads a bit like a novel.
Edit:
Whoa, gilded! And I was already happy that I could provide an answer that wouldn't be deleted.
Also the French resistance, glider-borne and paratroopers cut telephone cables which made it harder for the Inland forces to contact the people on the coast.
Weather was also a factor since a short break in the poor weather was predicted by the Allies but not the Germans, due to weather stations in the Atlantic.