Jamieson, Fausset and Brown on 1 Corinthians 15:6

6. five hundred--This appearance was probably on the mountain (Tabor, according to tradition), in Galilee, when His most solemn and public appearance, according to His special promise, was vouchsafed (Mt 26:32; 28:7, 10, 16). He "appointed" this place, as one remote from Jerusalem, so that believers might assemble there more freely and securely. Alford's theory of Jerusalem being the scene, is improbable; as such a multitude of believers could not, with any safety, have met in one place in the metropolis, after His crucifixion there. The number of disciples (Ac 1:15) at Jerusalem shortly after, was one hundred and twenty, those in Galilee and elsewhere not being reckoned. Andronicus and Junius were, perhaps, of the number (Ro 16:7): they are said to be "among the apostles" (who all were witnesses of the resurrection, Ac 1:22).

remain unto this present--and, therefore, may be sifted thoroughly to ascertain the trustworthiness of their testimony.

fallen asleep--in the sure hope of awaking at the resurrection (Ac 7:60).