As somebody who is a very active hitchhiker in Germany and Europe, I feel like I should add a couple of things that have not yet been mentioned – especially for people who stumble upon this now.
Hitchhiking in Germany is fairly beginner-friendly. My main advice for Germany would be to try to get onto the highway and when you do, ask your driver to take you to a service station. Getting onto the highway is usually the hardest part by far, so accepting rides into a city (even if it's further away than the next service station) is usually a bad idea – unless it's getting late and you want to take public transport to your final destination.
In my experience, it's usually easier to find a ride by talking to people as opposed to using a sign or thumb, provided you find a good spot to do so. In some locations, signs can also be very effective.
One more tip: Always trust your instincts, resources, and planning more than driver's advice. I usually got very stranded, when drivers drove me to places about which they said something like "you'll have an easy time there," unless they were a hitchhiker themselves.
Now, to answer your questions:
Am I going to be more successful on a weekday or on the weekend?
Weekdays have far more traffic and hence better chances. I often get picked up by business-people on work-trips, which rarely happen on weekends. Also, you have better chances of hitchhiking with truck drivers (works best if hitchhiking alone).
Is there a difference between Saturdays and Sundays?
Yes! On Sundays and on statutory holidays commercial truck ops are forbidden until 10pm (few exceptions apply). Furthermore, in July and August trucks cannot take certain routes on Saturdays, too (only 7am till 8pm). Your hitchhiking options might be a bit more limited then. Beware, holidays differ from state to state.
And what time of the day would you recommend?
Daytime tends to be easier but I've also had success at night. That's partly because there's more traffic during the day, and also because people tend to have an easier time trusting you.
A word about different highway infrastructure options:
- When looking for service stations, try to find ones that only service
one direction of travel flow, and ones that can't be publicly accessed from
local roads. You're mainly looking for the word Raststätte,
those are big service stations where many people stop for gas, food,
and toilets. If you're travelling alone, you also have great chances
of finding long-distance rides with truck drivers.
- Sometimes you can go for rides to a Rastplatz. Those are less
frequented, as they only have toilets and some tables with benches,
and people usually only stop shortly to go to the toilet or to have a
quick snack they brought themselves. I usually only go for this if I struggle to find any better rides, or if the other option would be to get dropped off at some random village.
- My least favorite option is so-called Autohof. Those are big service stations with multiple gas stations and restaurants that are close to the highways. They come with some disadvantages, as they service both directions of traffic-flow on the highway in addition to local traffic. Hence your chances of finding a ride in the correct direction when asking around are only about 1/3. This can be quite frustrating. It should be mentioned, though, that they come with relatively high traffic-flow and if you find a good spot to stand with a sign, you might get lucky.
Some resources you might find helpful:
- When looking for good spots in general, consult this public and open-source map. The countless user reviews really give you a good feel for the spots.
- When looking for service stations, look at this commercial map, maintained by the companies who run them.
- When looking for any hitchhiking related advice, go to this Wiki. They have great entries about most major cities worldwide, as well as country-specific tips and tricks.