Menu planning does not necessarily have to happen on a Monday; you can choose any day of the the week as long as you consistently use the same day of the week.
I personally like to menu plan on Sunday when I do my other weekly plans. I feel it gets me ready for the week.
And menu planning is easier if you follow this outline in this order:
–Day 1 thru 7—
1. Meat dish . . . decide 7 day’s worth before moving on to veggie/sides
2. Veggie/Side #1
3. Veggie/Side #2
4. Veggie/Side #3
5. Lunch and Breakfast (often incorporates a part of leftovers)
6. Desserts, occasionally
Once you get the hang of it, menu planning becomes easier.