What counts as period day one?

Jennifer

So my last couple periods have started out super light. This is my second one since having my baby almost a year ago. I only see it when I wipe, not on my underwear. Definitely not enough to need a pad/tampon, maybe a liner. Does day one count when the flow is heavy enough for a tampon? Or as soon as I start spotting? Why in my 13 years of having a period I’ve never struggled with this is beyond me 😂