worse is that the text tells readers that Mr. Hooper has always previously been a bright and cheery person at weddings. He is the kind of person whose very presence calms and encourages the nervous couple while at the same time conveying a general cheerfulness to all the wedding's attendees:

That night, the handsomest couple in Milford village were to be joined in wedlock. Though reckoned a melancholy man, Mr. Hooper had a placid cheerfulness for such occasions, which often excited a sympathetic smile where livelier merriment would have been thrown away. There was no quality of his disposition which made him more beloved than this.

When Mr. Hooper shows up at the wedding, he does not usher in the standard happy wedding feelings. Instead, readers are told that Mr. Hooper brings in an overall sense of evil:

When Mr. Hooper came, the first thing that their eyes rested on was the same horrible black veil, which had added deeper gloom to the funeral, and could portend nothing but evil to the wedding.

We are told that the people in attendance feel as if the room has grown darker. The bride becomes so pale that people think that she looks like the dead body from the day's earlier funeral. We are specifically told that Mr. Hooper's black veil creates one of the most "dismal" weddings ever.

The wedding scene in Nathaniel Hawthorne's "The Minister's Black Veil" is greatly affected, particularly when it comes to how the atmosphere of the event went from joyous to macabre.

The narrative tells of how the veil brought with it an overall sense of death and desolation marked by the "cold fingers" of the bride, as well as by the tremulous looks of the guests.

When Mr. Hooper came, the first thing that their eyes rested on was the same horrible black veil, which had added deeper gloom to the funeral, and could portend nothing but evil to the wedding.

The description of the sensation produced by the veil consists in how it compares to a "cloud" that have "rolled duskily" bringing everything to a halt, and thus darkening the mood of an otherwise happy occasion. Lest we mention that the minister himself had an epiphany during the wedding as he raised his glass of wine when he saw his own reflection. Even he was perplexed by what he saw behind his own black veil, which was himself and how he looks like to the whole world.

In the sin-obsessed atmosphere of the Puritan era, the Reverend Mr. Hooper's black veil, which hides the lower portion of his face and casts doubt in the minds of his congregation that their minister may be hiding some dark secret, casts a portentous gloom upon the wedding ceremony when he arrives.

This congregation of Puritans who carry an unforgiving view of human nature wonder at their earlier church service what their minister is hiding behind "his two folds of crape [crêpe]." The "gloomy shade" that Mr. Hooper continues to wear to a funeral and now to a wedding is very unnerving to the people.

Each member of the congregation, the most innocent girl, and the man of hardened breast, felt as if the minister had crept upon them, behind his awful veil, and discovered their horrible iniquity of deed or thought. 

Thus, when the minister arrives at the wedding wearing the dark veil over his face, the mood of the gathering is greatly affected. Whereas the guests have supposed that the minister would discard his veil and adopt his usual "placid cheerfulness" for this happy ceremony, he instead enters wearing this small pall-like cloth. Thus,

...a cloud seemed to have rolled duskily from beneath the black crape and dimmed the lights of the candles.

To the wedding guests it appears that the pale bride resembles the maiden who has been buried only a few hours before, and this deathlike maiden is one and the same.
So powerful an effect has this black veil of the minister produced that even when he catches a glimpse of himself in a looking glass, he himself shudders with horror, spills his untasted wine, and rushes out into the dark night. "For the Earth, too, had on her Black Veil."

The minister's black veil casts a pall upon the occasions Mr. Hooper attends so that the funeral differs little from the wedding. From his donning of this veil, then, there evolves a feeling of dread in the congregation whenever they encounter the Reverend Mr. Hooper, on whose face they can only detect the "glimmering of a melancholy smile," and he is avoided. 

The black veil that Mr Hooper dons has a massive impact on the wedding. Remember that this is the first day that he has worn the black veil, and the congregation think that after the funeral service, which of course was an appropriate service to wear a veil to, Mr Hooper will reconsider his attire for the wedding. However, he does not, and it is clear from the narrator's comments that the guests and the bride and groom themselves are affected greatly by the veil. Note the following description:

Such was its immediate effect on the guests that a cloud seemed to have rolled duskily from beneath the black crape, and dimmed the light of the candles.

Not only does the black veil therefore apparently darken the area around Mr Hooper in the church, even going as far as to "dimming" the candlelight, it also turns the bride into a very pale and trembling figure, so much so that some members of the congregation believe her to be the young woman whose funeral was celebrated just a few hours before. The black veil therefore is seen as a bad portent for such an occasion, and it clearly affects the ceremony.

While the minister’s black veil does not seem so very out of place at the funeral over which Mr. Hooper presides earlier in the day, it seems incredibly austere and even grotesque at the wedding of the “handsomest couple” in the village. This is a joyous occasion—or rather, it should be—but Mr. Hooper’s strange veil casts a pall over the entire proceedings. The crowd who assembles for the ceremony and festivities feels that the “horrible” veil could “portend nothing but evil to the wedding.” A “cloud” seems to fog the room, even appearing to dim the lit candles, and the bride shivers and shakes when faced with the minister’s shrouded visage. In her evident fear, her skin is of such a “deathlike paleness” that onlookers actually compare her to the dead maiden who was buried earlier that day.

The narrator describes the wedding as positively “dismal,” and this feeling is all as a result of the black veil. Even at the reception, when Mr. Hooper raises his glass to toast the newlywed couple, he sees himself in a mirror and is so affrighted by his own reflection that he turns white, drops his wine glass to the floor, and runs out into the night, “for the Earth, too, had on her Black Veil.”