Every time you sub.
Teachers loves notes. Teachers love to know how things went in their classroom and unless you are going to be able to talk to them the next day, a note is the only way to communicate what happened. I have been asked to come back to the class again because I wrote a note (the teacher loved it so much) and her team teachers asked me to sub. Yeah for pre-arraged jobs!
To make things easy for me I usually write a little blurb about each subject for the day as it is laid out in the sub plans. For example I would write Math then talk about what happened in math, then I would write Writing and talk about writing and so on. I always include what was finished or not finished and where to find any papers that I collected or if students still have them.
At the end of the note I add more general things such as a super helpful person (para, teacher, parent), if there were notes given to me and where they are, any messages I received for the teacher.
Any behavior problems that I have had that were already taken care of don’t make it to the note. No teacher wants to come back after being gone and read two pages about their students not listening. Major behavior problems always get communicated to the teacher.
Don’t forget to let them know you would like to come back (if you do) and leave your contact information for them.
Just for extra fun I usually leave a piece of chocolate for them too.