In my paper I focused on the anniversary of the Romanian "Great Union" (The Declaration of Alba Iulia, 1. 12. 1918) and of the Hungarian signing of the Trianon Peace Treaty (4. 6. 1920). Although these two anniversaries are closely connected, they play very different roles in the national narratives and in the collective memories of Romanians and Hungarians. No wonder that also the ways and aims of their commemoration are significantly different. First, I tried to explain in short what had happened on those two dates a century ago, then I sum up the history of the two anniversaries along the 20th century, and finally I took a look at their commemoration and instrumentalization in the present, with the occasion of the centenary.