Why is today a Holy Day of Obligation! Here is St. Bernard with the answer:
Calling the saints to mind inspires, or rather arouses in us, above all else, a longing to enjoy their company, so desirable in itself. We long to share in the citizenship of heaven, to dwell with the spirits of the blessed, to join the assembly of patriarchs, the ranks of the prophets, the council of apostles, the great host of martyrs, the noble company of confessors and the choir of virgins. In short, we long to be united in happiness with all the saints.