Gerd Luedemann, The Resurrection of Jesus: History, Experience, Theology (Minneapolis, MN: Fortress Press, 1994), 107.
Conclusion: The appearance before the ‘more than 500’ [1 Corinthians 15:6] as a historical phenomenon can plausibly be represented as mass ectasy which took place in the early period of the community. Given the nature of mass psychology, the stimulus towards it may have been provided by one or more individuals. Again that fits well with what has been worked out so far, namely that at least a first appearance took place to Peter (and the Twelve). Here we may pursue this notion just a little further: Peter saw the crucified Jesus alive (as did the Twelve). They also spoke of it, for example, at the next great festival (after the Passover at which Jesus died) in Jerusalem, the Jewish Feast of Weeks (=Pentecost), on which many festival pilgrims met. (Indeed it was such a festival which first made possible the appearance to a large number of people.) This preaching and the recollections of Jesus which were generally present formally led to a religious intoxication and an enthusiasm which was experienced at the presence of Jesus, indeed as the presence of the Risen Christ as Peter had seen him.
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The Amateur Exegete 