Are you amusing and enjoying yourself, and uttering witticisms and raising a laugh, and regarding the matter as a mere nothing? How many perjuries, how many injuries, how many filthy speeches have arisen from witticisms! "But no," you will say, "pleasantries are not like this." Yet hear how Saint Paul excludes all kinds of jesting. It is a time now of war and fighting, of watch and guard, of arming and arraying ourselves. The time of laughter can have no place here; for that is of the world.
Be ye therefore followers of God, as most dear children; And walk in love, as Christ also hath loved us, and hath delivered himself for us, an oblation and a sacrifice to God for an odour of sweetness. But fornication [Πορνεία, Porneia], and all uncleanness [ἀκαθαρσία, akatharsia], or covetousness [πλεονεξία, pleonexia], let it not so much as be named among you, as becometh saints: Or obscenity [αἰσχρότης, aischrotēs], or foolish talking [μωρολογία, mōrologia], or scurrility [εὐτραπελία, eutrapelia], which is to no purpose; but rather giving of thanks. For know you this and understand, that no fornicator, or unclean, or covetous person (which is a serving of idols), hath inheritance in the kingdom of Christ and of God. Let no man deceive you with vain words. For because of these things cometh the anger of God upon the children of unbelief. Be ye not therefore partakers with them. Ephesians 5:1-7
Per Saint John Chrysostom:
Let there not be one idle word; for from idle words we fall also into foul words. The present is no season of loose merriment, but of mourning, of tribulation, and lamentation: and do you play the jester? What wrestler on entering the ring neglects the struggle with his adversary, and utters witticisms? The devil stands hard at hand, roaring like a lion (1 Peter 5:8).
Hear what Christ says: The world shall rejoice, but you shall be sorrowful (John 16:20). Christ was crucified for your ills, and do you laugh? He was buffeted, and endured so great sufferings because of your calamity, and the tempest that had overtaken you; and do you play the reveller? And how will you not then rather provoke Him?
For indeed this is a work of the devil, to make us disregard things indifferent. First of all then, even if it were indifferent, not even in that case were it right to disregard it, when one knows that the greatest evils are both produced and increased by it, and that it oftentimes terminates in fornication. The man then who deals in jests is no saint. Nay, were he even a Greek, such a one would be scorned. These are things allowed to those only who are on the stage. Where filthiness is, there also is jesting; where unseasonable laughter is, there also is jesting. Jesting renders the soul soft and indolent. It excites the soul unduly, and often it teems with acts of violence, and creates wars. But what more? In fine, have you not come to be among men? Then put away childish things. (1 Corinthians 13:11).
If the thing is creditable, why is it left to mountebanks? Do you make yourself a mountebank, and are not ashamed? Look at your merriment-makers, those buffoons. Those are your mountebanks, peddling their gratifying deceptions. Theirs is an enormity of baseness and the emptiness of all reverence.
St. Thomas Aquinas inherited his thinking of eutrapelia from Aristotle (Ethic. iv, 6-8). Now, Saint Aquinas writes that play goes beyond the rule of reason by its species - being discourteous, insolent, scandalous, or obscene - or by its excess at time or place. But, he also writes that it is against reason for a man to be burdensome to others by offering no pleasure or by hindering their enjoyment (Second Part of the Second Part, Q. 163, A. 3-4).
Excess play is a mortal sin if a man prefers its pleasure to the love of God, so as to be willing to disobey a commandment of God or of the Church; it is a venial sin if a man is not willing to disobey a commandment of God or of the Church. Lack of play is, and can only be, a venial sin, as play is, not for its own sake, but for the sake of operation.
The vast majority of entertainment is therefore sinful by its species, and mortally sinful as it disobeys the commandments of God and of the Church. And to partake in such entertainment is against the commandment of God and of the Church, and is therefore itself mortally sinful.
It as well bears the danger of falling into the discourtesies, the insolences, the scandals, and the obscenities that such entertainment depicts. For men imitate. Hence Jesus said to his disciples: If any man will come after me, let him deny himself, and take up his cross, and follow me (Matthew 16:24).