The main idea behind The Feast of the Saints comes from an ancient eastern celebration coming just after Pentecost. Western Christians moved it to October, and finally to November by the 1200s. It really began when St. Helena converted the pagan Pantheon (“All Gods”) temple in Rome to a Catholic Church dedicated to all the martyrs - to whom was applied the label “The Saints” most commonly in the 300s.
It is the same with September 29 being the feast of St. Michael the Archangel for a long time. This was because Sept. 29 was the day on which the basilica of St. Michael was dedicated by a Pope a very long time ago. As with All Saints Church in the Pantheon, this and many other dedication feasts became universal “thematic” feast days. A lot of our universal feasts come from local Roman church-dedication days.
On the other subject, Grace is always available to the person who is united with Christ by baptism, love, prayer, forgiveness, and a life of virtues. Even for people who are unbaptised, unloving, prayerless, judgmental, and vicious, Grace is constantly working. I don’t believe grace is more available on “big”/extraordinary feasts compared to “little”/ordinary days.
God is present in every time and place. Now, that is not to say that special graces are unavailable on special days, given by the Church as the very Body and Heart of Christ. For example, on All Souls Day a special grace is given for the living to fulfill and give a plenary indulgence of mercy for those who have died. This exists only on November 2, and the 8 days following it.