In Fantastic Beasts: The Crimes of Grindelwald we found out that Grindelwald and Dumbledore
made a blood pact so they cannot fight each other. The memory Albus saw in the mirror implies that they did it out of friendship.
But it seems to me, that there is no time when this could have been made. In the time of the movie Ariana Dumbledore is already dead (Albus said that to Leta in the Hogwarts).
We know that Ariana died in a fight between Grindelwald, Dumbledore and Aberforth as described by Aberforth, i.e. on Pottermore site about Ariana:
I pulled out my wand, and he pulled out his, and I had the Cruciatus Curse used on me by my brother’s best friend – and Albus was trying to stop him, and then all three of us were duelling
So at that time
the blood pact couldn't be already effective. They obviously could fight each other.
But on the other hand, it would be strange if
they would make this pact after Ariana's dead, they weren't friends anymore after this event.
So, the question is obvious:
When was the pact between Grindelwald and Dumbledore made?