Yes, International Literacy Day is typically capitalized as it is a proper noun referring to a specific day of observance.

Maya Angelou was appointed to the National Commission on the Observance of International Women's Year by President Jimmy Carter. This council was established in 1975 to promote gender equality and women's rights.

International Mother Language Day is an observance held annually on 21 February worldwide to promote awareness of linguistic and cultural diversity and multilingualism. It was first announced by UNESCO on 17 November 1999. Its observance was also formally recognized by the United Nations General Assembly in its resolution establishing 2008 as the International Year of Languages.

International Public Relations are commonly conducted with special regard and observance of the cultural, protocolar and customs particularities of the region of the world where an IPR event or mission is conducted.

The plural of the noun observance is observances.