(If you are interested, this search result page gives links to the other volumes - or you can simply change the numbers I-IV in the URL.)
The proper reading given for the feast is from the writings of Scotus himself. I was going to summarise it for you but that is impossible: this is Scotus after all. So here is a translation:
Charity is called a habit by which God is held dear. But God could be held dear by some private love by which the one loving did not wish to have a co-lover (as in jealous men who have beloved women). But this habit would neither be ordered nor perfect.The practical lessons of this passage are that we should love God above all things and that when we love others, we should seek for their good, not treat them as some way of satisfying ourselves. And their good will always be to love God so all our friendships should tend in the direction of helping one another to love God more.
It would not be ordered because God, who is the common good, does not wish to be the private good of someone; nor according to right reason should someone appropriate to himself this common good. And therefore that love which inclines to this good as to a proper good, not to be co-loved or had by another, would be disordered.
It would be imperfect because one who loves perfectly wants the beloved to be loved; therefore God, infusing the habit by which the soul tends to him in an ordered and perfect way, gives the habit by which he is held dear as a common good, and one to be co-loved by others. And thus that habit which is of God, tends also to want him to be held dear and to be loved by another.
Therefore as that habit inclines to loving God in himself in an ordered and perfect way, it thus tends to want him to be loved by himself and by anyone else whose friendship is pleasing to him.
From this it is clear how the habit of charity is one, because it does not first look towards many objects but looks towards God alone as the first object as the first good in himself; and secondarily to want him to be loved and through love to be had by anyone, as he is in himself, because in this is the perfect and ordered love of him. And by willing this, I love myself and my neighbour out of charity, willing for me and for him to want God, and through love to have God, in ourselves.
It is clear therefore that by the same token I want God and I want you to want God. And by this means I love out of charity because according to this I want for you a good of justice.
And accordingly the neighbour is not assigned as though as a second object of charity but as though an entirely accidental object, because it is as something able to co-love the beloved with me in a perfect and ordered way; and for this I love him that he should co-love. And in this I love him as though accidentally, not because of him but because of the object which I want to be loved by him. And in willing this object to be loved by him, I implicitly will a good for him because it is a good of justice.
The Collect for the feast is as follows:
Domine Deus, fons omnis sapientiae, qui Beatum Ioa´nnem presbyterum, Immaculatae Virginis assertorem, nobis magistrum vitae et scientiae dedisti, concede, quaesumus,I was going to do an old-style-ICEL translation of that but perhaps I can leave that to you.
ut, eius exemplo illuminati, et doctrinis nutria, Christo fideliter adhaereamus. Qui tecum vivit.
O Lord God, fount of all wisdom, who gave us the Blessed John the priest and defender of the Immaculate Virgin as a teacher of life and knowledge, grant we pray you, that enlightened by his example and nourished by his doctrines, we may cling faithfully to Christ. Who lives and reigns with you...