Why evening is the common choice
Magnesium is often taken in the evening because some people find it helps them feel more relaxed before bed. There is no single perfect minute — the more useful habit is taking it at roughly the same time each night so it becomes automatic.
If mornings are when you remember supplements best, that is fine too; steadiness beats an ideal clock time you keep missing.
Making it a habit that sticks
Anchor it to something you already do every night, like brushing your teeth. A reminder you can check off straight from the lock screen keeps the routine from slipping on tired evenings. Pillog keeps everything on your phone with no account, and shows your month as a single card so you can see how consistent you have been.
Everyone responds differently, so if you are unsure about type or timing, your doctor or pharmacist can point you in the right direction.