For Oregon, State Scholastic Championship tournament. Usually happens in April.
Can be found here: https://www.calendarwiz.com/calendars/calendar.php?crd=nwsrsevents&&jsenabled=1&winh=587&winw=1280&inifr=false
For National Scholastic Tournament, below are the most prestigious ones:
National K-12 Grade Championships. Usually happens in Dec.
National Middle School (K-8) Championship. Usually happens in April.
National Elementary (K-6) Championship. Usually happens in May.
All-Girls National Championship. Usually happens in April.
Details can be found here: https://new.uschess.org/national-events-calendar
Above are the open tournaments, which means anyone who qualifies by the age/grade/gender can attend.