"It's a name that can open this door night or day, lad!—and welcome!"
Why has Mr. Welshman's attitude changed, and what is "unusual" about this statement? The quote is found at the beginning of Chapter 30.
In Mark Twain's The Adventures of Tom Sawyer, Huck has been listening to Injun Joe and his partner plan the mutilation of the Widow Douglas—as they stand hidden right outside of her home. Because she has been kind to Huck, he feels compelled to protect the Widow from harm. He sneaks away so he does not make a sound and alert the "villains" to his presence, and runs to the Welshman's house. When he franticly knocks on the door, the old man (the Welshman—Mr. Jones) asks who it is. When Huck answers, his welcome is not a warm one. The Welshman says:
Huckleberry Finn, indeed! It ain't a name to open many doors, I judge!
Seeing how upset Huck is with the news he holds, the old man lets him in to hear his story—about the threat to the Widow Douglas. The Welshman and his sons arm themselves and race to the Widow Douglas' home. Huck waits nearby until he hears "an explosion of firearms and a cry." As quickly as possible, Huck runs away, fearful that Injun Joe will learn that Huck was involved in bringing aid to the Widow, and kill him. This ends Chapter Twenty-Nine.
In Chapter Thirty, when dawn comes, Huck again stands outside the Welshman's door, knocking. He identifies himself...
Please let me in! It's only Huck Finn!
Huck's reception is very different than the day before in two ways. This time the Welshman has "changed his tune," and he tells Huck:
It's a name that can open this door night or day, lad!—and welcome!
Not only does the Welshman seem pleased that Huck is there (which we find out is because Huck went to such lengths to protect the Widow Douglas), but he uses the word "welcome" with Huck. Huck is surprised, not remembering ever in his lifetime that someone had expressed such a positive response to Huck—no one ever made a move to "welcome" him.
These were strange words to the vagabond boy's ears, and the pleasantest he had ever heard. He could not recollect that the closing word had ever been applied in his case before. The door was quickly unlocked, and he entered.
The speed with which the door is opened also indicates that a warm welcome does, in fact, wait for Huck within the Welshman's home.