Anne Frank kept a diary from 12 June 1942 to 1 August 1944. Initially, she wrote it strictly for herself. Then, one day in 1944, Gerrit Bolkestein, a member of the Dutch government in exile, announced in a radio broadcast from London that after the war he hoped to collect eyewitness accounts of the suffering of the Dutch people under the German occupation, which could be made available to the public. As an example, he specifically mentioned letters and diaries.
Impressed by this speech, Anne Frank decided that when the war was over she would publish a book based on her diary. She began rewriting and editing her diary, improving on the text, omitting passages she didn't think were interesting enough and adding others from memory.