You may have received an invitation to claim your HarvardKey. If not, you will be getting it someday.
HarvardKey is Harvard's move to one ID and password for a bunch of Harvard sites. Instead of having to remember an ID and a password for each Harvard facility, you will only need to remember one HarvardKey ID and password for all of them. Or that is how it will work when they all get set up for HarvardKey. (Your ID and password for this website will remain unchanged.)
The rules for your password are tricky. It is for a good cause, to deter hackers, and the rules are similar to those used by some other sites. Sooner or later you will want a HarvardKey, so you might as well get it when you receive the invitation.
Caution! There is a link you can click to convert your post.harvard.edu email to an alumni.harvard.edu email. If you click on that, everything you have subscribed to on that ID will no longer reach you, and no one will be able to send you an email on post.harvard.edu any more. There is no way to reverse that change. My recommendation is do NOT click that link.