week 1 feed schedule

Hi, this is my first time growing, so Ive been looking to find out when I should feed my seedlings in pre-buffered coco (im using canna coco professional plus).. Some people say to wait until yellowing of the cotyledons and others say to feed straight away with low strength.. Which is...

I just switched the lights to 12/12 last Tuesday. Would last Tuesday be week 1 or tomorrow? For following a feed schedule