| May 20th
The supernova in M101 was discovered by Koichi Itagaki, on May 19th.
His discovery image:
I obtained this image the next day.
The supernova was at magnitude 11.7
Some indication of the immense power of a supernova can be seen from this chart -
the supernova peak far outshines the galaxy's central core which could contain millions if not billions of star.