For some time he had become very strange

"You’ll remember me, I’m telling you truly."

Tomorrow, December 23, is the anniversary of Vincent Van Gogh cutting off part of his ear and giving it to a prostitute. The circumstances of that night are not clear. He had been living in Arles, France with Paul Gauguin in an attempt to form an artists colony, but their relationship had soured. 

That night they were both drunk and argued. It is thought that they were arguing about the local prostitutes, who preferred Gauguin to the brooding and erratic Van Gogh. What ever the truth to that story, no doubt the deeper conflict was over Gauguin's decision to return to Paris.  

The website Vincent Van Gogh, the Letters has an archive of Van Gogh's correspondence along with detailed notes about the individual letters. In January Vincent wrote to his brother Theo about his recovery. The attached notes mention that Gauguin told a friend of his what had happened shortly after returning to Paris.  This seems to be the most reliable version Gauguin gave of the night (paragraph breaks added for clarity):

Gauguin returning precipitately, 4 days ago, and the news about Vincent in the hospital. I rushed to see Gauguin, who told me this. On the eve of my departure (because he was about to leave Arles) Vincent ran after me (he went out, it was at night). I turned round, because for some time he had become very strange, but I mistrusted him.

Then he said: You are silent, but I shall be so too. Ever since I had been going to leave Arles he was so odd, I couldn’t live any longer. He had even said to me: “Are you going to leave?” And since I had said “Yes” he tore this sentence out of a newspaper and put it into my hand: “the murderer fled”.

I went to sleep at the hotel, and when I returned the whole of Arles was outside our house. Then the gendarmes arrested me, because the house was covered in blood.

This is what had happened. Vincent had returned after I left, taken his razor and clean sliced his ear. Then he had covered his head with a tall beret and had gone to a brothel to bring his ear to an unfortunate creature, saying to her: You’ll remember me, I’m telling you truly. This young woman fainted on the spot.

The gendarmes set out, and they all came to the house. Vincent was put in the hospital. His condition is worse, he wants to sleep with the patients, chases after the Sister and washes himself in the coal bunker. In other words, he is performing biblical mortifications. They were compelled to put him in a private room. 

Also from the notes, Vincent's brother Theo visited him in the hospital shortly after the incident. This is how he descrubed it to his wife Jo:

I found Vincent in the hospital in Arles. The people around him realized from his agitation that for the past few days he had been showing symptoms of that most dreadful illness, of madness, and an attack of fièvre chaude, when he injured himself with a knife, was the reason he was taken to hospital.

Will he remain insane? The doctors think it possible, but daren’t yet say for certain. It should be apparent in a few days’ time when he is rested; then we will see whether he is lucid again. He seemed to be all right for a few minutes when I was with him, but lapsed shortly afterwards into his brooding about philosophy and theology.

It was terribly sad being there, because from time to time all his grief would well up inside and he would try to weep, but couldn’t. Poor fighter and poor, poor sufferer. Nothing can be done to relieve his anguish now, but it is deep and hard for him to bear.

Had he just once found someone to whom he could pour his heart out, it might never have come to this. In the next few days they will decide whether he is to be transferred to a special institution and as I don’t yet know how much I shall have to do in all this, I dare not make any plans.

Although he was released from the asylum, Vincent never really recovered from that night. From that point on his mental state slowly deteriorated until he committed suicide less than two years later.

Vincent Van Gogh, the Letters is a good resource if you're interested in his life. Not only can you search the letters, but there is also a large amount of notes and supplemental information that puts them in context. Finally, letters that have sketches can be listed separately, and there are facsimiles that show the drawings. Seeing the pencil and ink sketches of Van Gogh is quite interesting.