Please consult https://welcome-services.desy.de/international_office/ for useful information to enter Germany and reach DESY.
The Campus map is available here, also with local transportation info to DESY: https://welcome-services.desy.de/international_office/maps_and_directions/
DESY hostel (https://welcome-services.desy.de/hostel_in_hamburg/) is a very convenient solution. Options for the registration form are: 
Hamburg has naturally a wide range of hotel choices with an effective public transport service.
Breakfast and lunch can be taken at the DESY café & Canteen.
The social dinner is on Monday 20th April at 7 PM at Hoppe's Restaurant on the Elbe bank: Övelgönne 6, 22605 Hamburg. Fish, meat and veggie options will be available. The dinner includes a main dish and one soft drink/beer.
Bus route: https://maps.app.goo.gl/Rw8WCFtbPQmxNgbJ7.
Bus tickets cannot be bought on the bus anymore. You have to buy it beforehand using the link https://www.hvv.de/en The best option is to buy a Day Ticket Hamburg AB.
