I saw a homily on the web site of a Catholic Church.  I doubt that what is written is correct.  Please see below the excerpt of the homily.  Do you agree with the writer? 

He mentions that satanists are able to diminish the graces that will flow to us on the Feast Day we are celebrating. 

I remember Jesus once said to St. Faustina "You know the power of my grace.  Let that be enough."  I believe that if God wants us to receive His grace, it will get to us.  But the sad thing is it is up to us to accept it or not. 

Here's the excerpt of the homily which I found on the Catholic Church's web site:
The night before many of our Holy Feast Days such as the Immaculate Conception, or the Birth of John the Baptist, or All Saints (that is, Halloween) satanists will have big celebrations.  For these events they usually want a lot of Consecrated Hosts.  From midnight to 3 am they will engage in all kinds of immoral behavior, sacrificial offerings to satan, and profaning and desecrating the Consecrated Hosts.  Midnight to 3 am corresponds to, and is the opposite of, the time Christ spent on the Cross, that is from 12 noon to 3 pm.  By means of these celebrations, the
satanists are able to counter act the saving merits of Christ and to DIMINISH THE GRACES THAT WILL FLOW TO US on the Feast Day we are celebrating.  And the demons will have greater strength in causing people to be tempted, or lead astray.  By the horrible evils they commit at night, satanists counter act the good that we do during the day. 

Thank you and God bless you. 

Answer by Fr.  John Echert on 1/15/2013:

The work of the devils can not equal or exceed that of God and they are only allowed to act so far as God allows, but I cannot rule out the possibility that for some good reason, divine providence allows some interference in the realm related to the supernatural dispensation of grace.  I am inclined to the view that the work of the devils may impede the reception of grace by humanity and cannot constrain its offer by God, since God cannot be constrained by evil.  An analogy may be found in the human realm.  A priest who wrongly celebrates the Mass, such as was commonly done in decades past, with such abuses as clowns at Mass and dancing women, likely diminishes the prevalence of graces that the faithful might otherwise have received, even though the Mass may be valid, even if illicit.