"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...

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:

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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...

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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:

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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.