Fertile window?

Em

So I'm using Clear Blue Advanced ovulation kit and today I got the first flashing smiley face🌞 (last cycle I had 4 flashies before static smiley)...

...Can anyone tell me if this is the first day of my fertile window or would it have started a few days ago when my apps told me the FW would start?❓❓

ie. Does oestrogen surge indicate start of FW or not necessarily?

Would love any help on this please 😊❤️🌻