On the AoC website you need to provide solutions for each day in a given year to see the next day’s problem, so you can’t instantly see all the problems from a given year unless you’ve previously completed them.
You may be able to find the problems elsewhere on the net.
All problems are available as long as their day has come even while the event is running. But the part 2 of each problem requires solving part 1 first.
I think it started in 2015.
On the AoC website you need to provide solutions for each day in a given year to see the next day’s problem, so you can’t instantly see all the problems from a given year unless you’ve previously completed them.
You may be able to find the problems elsewhere on the net.