According to Wikipedia:

Nuclear power in Germany accounted for 13.3% of German electricity supply in 2021, generated by six power plants, of which three were switched off at the end of 2021, the other three due to cease operation at the end of 2022 according to the complete nuclear phase-out plan of 2011.

Germany has been planning to stop generating electricity from nuclear power for 20 years. As ABCNews in its article Germany shuts down half of its 6 remaining nuclear plants inform:

“The decision to phase out nuclear power and shift from fossil fuels to renewable energy was first taken by the center-left government of Gerhard Schroeder in 2002. His successor, Angela Merkel, reversed her decision to extend the lifetime of Germany’s nuclear plants in the wake of the 2011 Fukushima disaster in Japan and set 2022 as the final deadline for shutting them down.”

However, the current situation in Ukraine and the potential for an energy crisis in Europe could change or delay Germany’s decision to shut down its remaining nuclear power plants.

Germany’s three active nuclear reactors as of February 2022 are:

Emsland
Isar 2
Neckarwestheim 2

Will a major nuclear power plant in Germany be operational on June 1, 2023?

This question will resolve as Yes if Wikipedia’s page on Germany's nuclear reactors reports any as “Operational” on June 1, 2023. If wikipedia does not report this information, or it is clearly inaccurate, credible media reports may be used instead.