Wa-lā siyyamā means “and especially” or “in particular.” It singles out one member of a larger class for special emphasis. Among the many enjoyable days, the poet highlights yawmun bi-Dārat Juljul.
Arabic grammarians recognize different case analyses after this expression; the reading in this printed passage has yawmun in the nominative. The essential semantic point is specification, so the case issue must not obscure the meaning of the chosen day.
Compare the ordinary expression, “There were many pleasant occasions, especially the one we remember most.” This emphasizes how one remarkable occasion stands out among many.