NATO was established on 4 April
https://otan.delegfrance.org/NATO-s-history
The North Atlantic Treaty, which established the North Atlantic Treaty Organization, was signed in Washington on 4 April 1949 by 12 countries: Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal, the United Kingdom and the United States. Article 5 of the Treaty defines the collective defence commitment: "an armed attack against one or more of [the Parties] in Europe or North America shall be considered an attack against them all".