The factory is in an advanced stage of completion (at least its first phase) so any date from Summer 2021 to Fall 2021 would hardly be a surprise.
There is also one additional bit of information. The indicative date is only for the Long Range AWD version of the car, while when you switch to the Performance version, there is no information.
We guess, that the Performance version will follow. When? Well, it depends on many factors, but probably from several months, up to a year later.
Customer deliveries of the Tesla cars usually start several weeks to 1-2 months after the production start. In the case of the Made-in-Europe (MIE) Model Y, it could be September maybe?
The Tesla Giga Berlin plant not only will bring Model Y to Europe, but also pioneer several new technologies, like: 4680 battery cells, structural battery packs, mega castings (front and rear part of the car) and an innovative paint process (Geico Taikisha).
Having an independent manufacturing plant in Germany will be also important from a flexibility point of view to address the needs of the local market, as well as import/tax, logistic and exchange rates.