You can use the writing prompt that just appears (the one for that exact date) when you go to the site OR you can access the calander (pick a day) at the top right of the screen and choose a day and start writing.
Yesterday I worked in a year 4 classroom where the students used the prompt on May 2nd. They were working on the 7 Steps to Writing Success - tightening tension skill and they wrote a portion of a story that showed the development of tension or suspense.
The students were so enthusiastic about the writing prompt and it really inspired some great written responses. I hope you will find it to be a useful tool to use.