I do not really keep a planner and I honestly do not recommend keeping one yourself. Unless you are planning to write about certain topics that you get paid to do – in that case by all means, do keep a planner with notes or bulletpoints jotted down on a small notepad/scribblepad or even on Word/Notepad on your own pc/laptop. What I usually do is, if I get a few ideas about the topics that I want to blog about, I create them on WordPress and save them as drafts because I know that I will come back to them later. This way I don’t forget said topics and also I have my set of blog posts all mapped out for the next couple of days.
The main reason why I say that a planner isn’t something I recommend is because it looses spontaneity. When I do plan these things it seems a bit forced for me and I kinda lose a bit of the excitement as I already knew a couple of days ahead what I was going to write about. Now the exception is for paid/sponsored posts which either I have the list of topics (depending on what the client wants, it can be a set of 5 to 10 per month) already emailed to me, so I do know in advance what it is I will be blogging about.
However the stuff that I write just for me because it is what I enjoy writing about, am passionate about or what interests and intrigues me is always more spontaneous. The best thing is to get the idea or think about ideas or search for ideas and the moment one clicks, I go to my blog’s tab and open up a new post blog – the button that says “Add New” – and I start typing. Depending on how easy it is to write about it, I may spend from 20 minutes to a whole day on it.