Every day you can gain a plenary indulgence under the usual conditions by spending half an hour before the Blessed Sacrament, by reading the scriptures for half an hour, by making the Stations of the Cross in Church, or by saying the Rosary either in Church or in a family or other devout group.
However there are many plenary indulgences given for particular occasions. Although I have reservations about some of the practical reforms Pope Paul VI carried out, his Apostolic Constitution Indulgentiarum Doctrina is a superb theological and spiritual exposition of indulgences. It is not difficult to read and I heartily recommend it if you never heard anything at Catholic school about indulgences except perhaps that they were sometimes sold in the eeeeevil Middle Ages (Boo! Hiss! He's BEHIND you!)
One point made by Pope Paul VI is relevant to the indulgences granted for particular days:
The aim pursued by ecclesiastical authority in granting indulgences is not only that of helping the faithful to expiate the punishment due sin but also that of urging them to perform works of piety, penitence and charity—particularly those which lead to growth in faith and which favour the common good. (n.8)In other words, the granting of indulgences is also used by the Church to encourage us to particular devotions that help our spiritual life, perhaps at the same time gently guiding us away from devotions along the lines of the five first Wednesdays Novena of the Holy Kneecap of St Petronilla which you have to photocopy and leave at the back of the Church or the prayer doesn't work.
Last year I resolved to prepare a list to remind myself to announce these indulgences. I am happy to share the list on the understanding that it is not an official document, simply a guide:
Plenary Indulgences for particular days.
For the official description of each work you need to consult the Enchiridion Indulgentiarum. (For convenience I have given the reference numbers.)
For some guidance on the conditions for gaining a plenary indulgence, you could consult my post: Plenary indulgences not impossible.
The list is primarily for the assistance of parish priests who get the point of indulgences. I would be very happy to think that it might save them a little time. Lay people can of course use it as a guide too, (but please bear in mind that I don't have unlimited time to respond to scrupulous emails.)
There are a few that I had not spotted before. One I particularly like is the indulgence given for visiting a shrine constituted by the competent ecclesiastical authority as an international, national or diocesan shrine. That would obviously include Lourdes and Fatima (but not Medjugorje.) In my own diocese, it would also include Aylesford and Ramsgate (the National Shrine of St Augustine of Canterbury.) You can gain the indulgence on the titular feast, on one day each year of your own choosing, and whenever you go there as part of a group pilgrimage. (The Enchiridion uses the fine word turmatim - which Lewis & Short gives as "by troops or squadrons.")
Another one often forgotten is that on the feast of St Peter and St Paul you can gain a plenary indulgence by using a pious object blessed by the Pope or any Bishop, adding a legitimate profession of faith. If people know this, they might ask the Bishop to bless rosaries when they meet him - or the parish priest might even arrange for the Bishop to give a public blessing of pious objects at the end of Mass when he visits.
I knew that you could gain a plenary indulgence by visiting the parish Church (and saying the Pater and Credo) on the titular feast or on the anniversary of the consecration of the Church. What I had missed is that you can also gain a plenary indulgence on the anniversary of the consecration of an altar. Now I must go and look up the date of the consecration of our altar. Fortunately I did some tidying of the parish archives earlier today so I know where to look.
UPDATE: the anniversary of the consecration of the High Altar at Blackfen is 9 December.