When do you count your first day?

Deanna • I`m Deanna! I have a beautiful 5 year old named Madalynn! Hoping to add to my family one day :)
Like if you aren't supposed to start until yesterday, but you've been spotting brown for three days before that, what day would you usually count as day one? I normally have irregular periods but anyway, but its very rare for me to do that! And it was just barely any there