First scan for IVF?!

Gaelyn • Rainbow baby Stevie Grace born Feb ‘18 after 3 losses. TTC number 2. Wife of Chris. Stepmom of Zoe (13) Pitbull mom. Lover of rainbows, sunsets, summer, adventure.
Hi ladies - quick question. My doc has instructed me to call the clinic the day AF arrives to arrange starting my first IVF cycle (yay!). What I'm confused about is that the documentation says I will go in for a scan on either day 1 or day 3 of AF. Does anyone know what the difference is and why I would scan/start on day 1 or 3?