Citation data of scientific publications are essential for different purposes, such as evaluating research and building digital library collections. In this paper, we analyze to which extent citation data of publications are openly available, using the intersection of the Crossref metadata and unpaywall snapshot as publication dataset and the COCI dataset as open citation data. We reveal that for 24.2% of the publications, the citation data is openly available, while for 16.6%, the citation data is closed. We find that the percentage of publications with open citation data has increased over the years. We observe that whether publications are published with open access has no influence on whether their citations are openly available. However, publications published in journals from the Directory of Open Access Journals (DOAJ) tend to have more citation data openly available than publications from other journals.