First off, congratulations on your acceptance to Clark and being offered the Presidential Scholarship.
At this same time last year, my daughter received a similar "early review" acceptance from Clark, along with the Presidential Scholarship. Like you, she had already had an on-campus interview prior to her acceptance. She wasn't able to make it to Scholars Day, but was told that she would still be eligible for the scholarship because she had interviewed on campus already. Sure enough, the Presidential scholarship was included in her formal acceptance package that she received in late March. While she didn't end up enrolling at Clark, I was still impressed with the school, itself, and the Presidential Scholarship, which--if I'm not mistaken--is $60K over four years, is quite generous. Like iamhere mentioned, it would probably be a good idea to call Clark's admissions office and confirm your eligibility for the Pres. Scholarship, whether or not you are able to attend Scholars Day.
Once again, congratulations and good luck!