I am generally a visual person and usually need to do things on actual paper vs on the computer. I have a paper and an electronic version of a calendar for exactly that reason (I would just have paper, but the computer version is easier to transport).
I used to just grab the nearest scrap of paper to jot down my menus on, but that became challenging when I either lost the paper or forgot to label it with the date and had no idea what paper was for what week. It also didn't often allow for space to make notes or required a second piece of paper for a grocery list (which then goes into my phone).
As I have done more menu planning, I have learned what I need and what I like as far as layout. Finally, I made my own menu planner + shopping list + notes sheet to use on a weekly basis. It's a combination of what works for me and things that other people created (and I changed) that I thought would also work for me.
I didn't label the days of the week because one of the things that frustrated me about all the planners made by others was they were all Mon-Sun and I didn't always function on the traditional Mon-Sun week. So I simply made it seven days and left space to add days/dates in the boxes if desired. There's also a spot at the bottom to list any snacks you might want to make or remember to pack in your lunch throughout the week.
I don't always plan out all three of my meals each day, but sometimes if I'm planning on making a large batch of something I like to have an idea of how many days it will last (like egg muffins for breakfast this week), or when I'm pretty sure there are not going to be leftovers from dinner I want to have an idea of what day so I can plan something else for lunch.
Some weeks I make a menu and then we don't follow it at all. Those are usually weeks that I end up in trouble when it comes to lunch. Poor dinner planning = no leftovers = no lunch!