The Portiuncula Plenary indulgence reminder comes from Rorate Caeli:
On August 2, the “Portiuncula indulgence“: under the usual conditions, recite the Credo and the Pater in any Minor Basilica (33, § 2, c), Cathedral (33, § 3, e), or Parish Church (33, § 5, b). (Enchiridion Indulgentiarum, 4th edition, al. concessiones.)
According to the usual allowance concerning indulgences linked to specific days, local Ordinaries may define if the indulgence is equally applicable on another day, or on the preceding, or (usually the case) on the following Sunday (Enchiridion, Normae, n.15)
Catholicism Pure & Simple 
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