What happens one night two months before the murder of Sir Darvers, and why is this this event important?

Some two months before Danver's death, Dr. Jekyll went to bed as himself...... and he woke up as Mr. Hyde.

Some two months before the murder of Sir Danvers, I had been out for one of my adventures, had returned at a late hour, and woke the next day in bed with somewhat odd sensations. It was in vain I looked about me; in vain I saw the decent furniture and tall proportions of my room in the square; in vain that I recognised the pattern of the bed-curtains and the design of the mahogany frame; something still kept insisting that I was not where I was.....

Yes, I had gone to bed Henry Jekyll, I had awakened Edward Hyde.

This event is important because it was the first time Hyde appeared without the use of Jekyll's potion.