In my novel, Kafka In The Castle, I fill in the 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. In reality, the Swiss Girl haunted him (pleasantly) all his life.
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.