1st day of period confusion

Steph

So my doctor wants me to go get my progesterone levels checked first day of my period.

Usually my first day of bleeding I don’t count as my period due to it being basically spotting.

I would usually count day one as my second day of bleeding as its heavier.

Question is do I go for the blood test first day of bleeding even though it’s spotting?

Or do I go tomorrow when it’s a full on period ?