Signing in at www.planbookedu.com takes you to the planbook you were last working in. If you keep arriving in last year's instead, the usual reason is a bookmark.
The fix
A bookmark saved while you were inside a specific planbook points at that planbook, and keeps going there however many new ones you create.
Change your bookmark to plain www.planbookedu.com — no path after it. From then on you'll land in whichever planbook you used most recently.
Switching planbooks any time
Click Planbooks in the navigation bar and choose the one you want. That also resets which planbook counts as most recent, so the next sign-in follows you.
The year calendar has a switcher too, if you have several planbooks on the go.
Keeping the list tidy
If old planbooks make the list hard to scan, archive them. They stay readable and drop out of the way — see Archive a planbook.
Related: Manage multiple planbooks