Where Is Everyone? The Holiday Was Yesterday.
Yesterday in Germany was Himmelfahrt (the day of the Ascension), and a national holiday, where all businesses, universities, and schools closed for the day. For the average American, the concept of having a national holiday on Thursday that's anything other than Thanksgiving is a bit weird, as many of our holidays are intentionally set to Mondays (Presidents Day, Martin Luther King Jr. Day, Labor Day). Having these holidays fall on Mondays means an automatic three day weekend, and (probably) helps to ensure that the work week is relatively uninterrupted by people taking extra mid-week days. Instead of working the one day between the holiday and the weekend, in Germany, many people take a Brückentag.
A Brückentag (bridge day) is when someone takes off work on the day between a holiday and the weekend, mostly referring to taking off a Friday or Monday to bridge the gap between a Thursday or Tuesday holiday (like May Day this year, which fell on a Tuesday).
Doing so is common enough practice that if you are a student studying in Germany, you may want to just confirm in advance that any classes you may have running on a day which could be considered a bridge day will be running, in case your professor has taken the day. If you work in Germany, and are thinking of taking the four day weekend in order to travel, it's worth bearing in mind that unlike taking off a Friday or Tuesday to make a four day weekend in the U.S., either the first or last day of your short holiday will be during a national holiday, so you may need to plan for some crowds on the trains or roads.