Passage: On 26 June 2018, Nainggolan signed with Inter Milan for €38 million (€24 million plus Davide Santon and Nicolò Zaniolo, valued at €14 million) until June 2022. He scored in his league debut on 1 September, helping the visitors defeat Bologna 3–0.On 3 October 2018, Nainggolan scored the equaliser in a 2–1 comeback win at PSV Eindhoven in the Champions League group stage to become the first-ever Belgian to achieve the feat in the competition for Inter.

Question: When did Nainggolan join Inter Milan?
26 June 2018