December 16 was the Vermont deadline for candidates to submit 1,000 signatures to get on a presidential primary ballot. Three Republicans submitted a petition, and thirteen Democrats submitted one. See the list from the Vermont Secretary of State’s web page.
Julian Castro didn’t have 1,000 valid signatures, but the law lets candidates in that situation have a chance to collect more signatures, as long as they had submitted at least 1,000 raw signatures by the deadline.