When does William Ashby first appear in The Witch of Blackbird Pond?
William Ashby first appears in the book after Kit's first Sunday Meeting in Wethersfield. After the church service, Rachel Wood introduces Kit to "Mistress Ashby...and her son, William." William is immediately "dazzled" by Kit's beauty, and Kit, who has been quite shocked and frustrated by the staid ways of the New England Puritans, "unconsciously...reward(s) him with the first genuine smile she had managed (that) morning." It had been an unspoken understanding that William was interested in courting Judith Wood, but after he sees Kit, his plans change. Without ever meaning to, Kit, by her appearance in Wethersfield, has changed the social dynamics of the young people there; upon seeing Kit, William drops his pursuit of Judith, and sets his sights on Kit instead.
The account of William Ashby's first appearance in the narrative occurs near the end of Chapter 5. Although it is on page 57 in my book, the 1986 Random House Yearling edition, page numbers do vary from edition to edition of any given work, so it might be more helpful if I tell you that the account can be found 2-3 pages from the end of that chapter.