RKI data (most credible view into the past): time series data provided by the Robert Koch-Institut (updated daily):

• 7-day incidence time series resolved by county based on RKI data can be found in more-data/.
• This is the only data source that rigorously accounts for Meldeverzug (reporting delay). The historical evolution of data points in these files is updated daily based on a (less accessible) RKI ArcGIS system. These time series see amendments weeks and months into the past as data gets better over time. This data source has its strength in the past, but it often does not yet reflect the latest from today and yesterday.