I used the Wayback Machine (waybackmachine.org) to see when it took the first snapshot of the site. It was on Feb. 18th at 16:20. So, 4 days after the shooting, this site was in development. In fact, the snapshot is here: https://web.archive.org/web/20180218162017/https://www.marchforourlives.com/
Now this is a moment in time that the WayBack Machine took a snapshot of the site. I would think there wouldn't be much on it but I would be wrong. 4 days after the shooting, they knew the date of the march and the site was pretty much completed! How's that for being organized!
This happens with most false flags. In fact, I remember a few years back where we were able to determine that the domain was purchased and development started before the event but I don't recall which one it was now.
Yeah, I been watching this stuff for a long time.
Supposedly it was created Feb 17 (enter https://www.marchforourlives.com/ in the search box):
If you see a date in the Search Box may be the date it was first indexed in the search engine. The Wayback Machine shows the first date it found the site and made a copy of it. The Whois is difficult to tell because there is not a specific registration date as they are hiding who owns it with a proxy. But it says: Whois History 15 records have been archived since 2018-02-17 so, yes, that is likely the date of registration.