Pack a rain shell for July.
Germany has no dry season. Rain is possible in every month of the year, and on the national average the two wettest months are June and July. Not November. Not March. The summer ones, with July averaging around 78 mm (3 in).
Thunderstorms do most of that work. A warm afternoon builds, the sky goes black over the beer garden, and twenty minutes later it’s finished. Then the evening turns lovely and everybody walks back outside.
The rest of the picture depends on where you’re standing, and it shifts more than the size of the country suggests.
What Changes as You Head South
The northwest belongs to the Atlantic. Winters are mild there, summers are cool, and the wind blows on most days. Hamburg and the North Sea coast rarely get very hot or very cold. They get weather instead, and plenty of it.
Head east and the Atlantic loses its grip. Brandenburg is the driest corner of the country, with hotter summers and colder winters than the coast gets.
The warmest corner is the Upper Rhine valley, the trench that runs from Basel up toward Frankfurt. Spring arrives there weeks before it reaches the north, and the vineyards along both slopes are no accident.
The Mittelgebirge (low mountain ranges), the wooded belt strung across the middle of the country, rewrite the rules locally. The Harz, the Black Forest, and the Bavarian Forest are wetter and noticeably cooler than the land at their feet. They hold snow when the valleys have none.
Then the Alps, which make their own weather. The piece worth knowing is the Föhn.
It’s a dry wind that spills down off the mountains and warms as it falls. It scrubs the air so thoroughly that the whole Alpine chain shows up on the horizon from Munich, sharp enough to pick out individual summits. Half of Bavaria blames its headaches on it the same afternoon. :-)
What Each Season Actually Does
Spring is slow and unreliable. March can do anything at all. April does all four seasons in one afternoon, and the warm spell in the middle of it fools the fruit trees most years. The turn you can trust comes in the second half of May.
Summer runs warm rather than hot. A normal lowland day sits at 22 to 26 °C (72 to 79 °F). The coasts stay a few degrees behind that, usually near 20 °C (68 °F). Stretches above 30 °C (86 °F) now turn up most years, which is newer here than it sounds.
Air conditioning is the gap Americans notice. Hotels usually have it. Older apartments often don’t, and German windows tilt open at the top because that is the cooling system. The nights drop far enough to make it work.
September is the quiet favorite. Warm days, cool nights, and the best light of the year. October turns, and by November the fog sits in the river valleys until lunchtime.
Winter is grey more than it is cold. Daytime temperatures in the lowlands sit between 0 and 5 °C (32 and 41 °F). A hard freeze at -10 °C (14 °F) is an event, not a season. December is the darkest month of the year, with roughly 40 hours of sunshine in the entire month. November gets more than that, whatever anyone here tells you.
Lowland snow is a lottery. Some winters hand you a week of it, others none at all. Snow you can count on means the Alps or the higher Mittelgebirge.
The Dates We Plan Around
German weather comes with a folk calendar, and parts of it still get used.
The Eisheilige (Ice Saints) run from May 11 to 15, five saints’ days in a row. Cold polar air arrives with them often enough that no gardener here sets out the balcony boxes before the last one has passed. The north gets the cold first and the south catches it two days later, because the air needs the time to travel.
The Schafskälte (sheep’s cold) lands somewhere between June 4 and 20. Polar air again, taking 5 to 10 °C (9 to 18 °F) off the thermometer for a few days. It’s named for the sheep, which have just been shorn and feel every bit of it.
June 27 is Siebenschläfertag, and the old saying claims the next seven weeks will copy that one day. The national weather service puts it more carefully. The pattern that settles in around the end of June tends to hold through early July. In the north the rule doesn’t work at all.
Then the Altweibersommer (old wives’ summer), a stable high that usually turns up in late September. It’s the reason September gets booked by people who have been here before.
When to Come
Late May into mid-June, and the whole of September. Those are the two windows I’d take, and I’d take September first.
Daylight swings hard this far north. In Hamburg the sun sets just before ten at midsummer and just after four in late December. Munich gets roughly an hour less daylight in June and an hour more in December.
Take a rain shell rather than an umbrella. The wind wins that argument most days, and on the coast it wins every day. ;-)
One rule covers the rest of it. Whatever the month, carry a layer you can take off and something that sheds water.
Dates are also a crowd question, and the school calendar decides that one. It’s laid out on German holidays. The mountain end of the weather sits on the German Alps, and the rest of the everyday basics are on Germany info.