In my novel, Kafka In The Castle, I fill in missing entries of his actual diaries. There are many days to fill, as he either did not write during these days, or he destroyed the record.
I do give him a brief recognition of Friday 13th. Kafka was not a superstitious person, and such things weighed on him lightly.
In reality, memories of the Swiss Girl he mentions (a teen he met and probably had an affair with) haunted him all his life. But pleasantly – oh, so pleasantly.
13 April 1917
I almost wrote down the year as 1913. That was the year I met the Swiss girl. And I remember her joking about Friday the thirteenth, and how we had missed it by just a day. She was superstitious – Christians seem to be. I wonder what precautions she is taking today. It will be three years and seven months since I saw her. Yet some of the things we did could have happened last week. I think that memory must be made of rubber. You can sometimes pull it toward yourself – and sometimes it snaps away like a shot. Causing as much pain.