In 1963, hundreds of thousands of people stood ashore by the Nieuwe Maas and the Nieuwe Waterweg to wave two ships, deGroote Beer and the Waterman goodbye. The ships transported thousands of Feijenoord fans to Lisbon where the club faced S.L. Benfica on 8 May 1963 in the European Cup semi-finals. The first leg, held in Rotterdam a month earlier, finished 0–0. Despite Feijenoord eventually losing the match 3–1, this turned out to be the start of the most successful period in the clubs history. Feijenoord won the double for the first time in their history in 1965, and managed to win another double a few years later in 1969. The 1965 title secured Feijenoord a spot in the 1965–66 European Cup, where they faced multiple cup champion Real Madrid C.F. on 8 September 1965. During the match, Hans Kraay had to leave the pitch injured after 31 minutes, without being substituted. He returned at the start of the second half and scored the goal which resulted in a 2–1 win. During the match, fans favourite Coen Moulijn was attacked by a Spanish defender. Moulijn then proceeded to chase the defender down the pitch, leading other players, and even fans who entered the pitch, to do the same. The referee could do nothing but to suspend the match at 2–1 in Feijenoords favour. Two weeks later, Real Madrid comfortably beat Feijenoord 5–0 and eventually won the European Cup that season.
Question: What month was the start of the most successful period in Feijenoord's history?
Answer: April 1963