Debut of the Tibble Twins
In the books, including "Arthur Babysits" that this was adapted from, the Tibble Twins and their grandmother Mrs. Tibble are humans. In the TV series, they are bears.
The first appearance of Arthur's great-grandmother whom is the maternal grandmother of his father Mr. Read, and the mother of Grandma Thora Read. She is shown again as a young woman in a flashback in "Clarissa Is Cracked" as a backstory of how Grandma Thora got her doll Clarissa since the age of 5.
During Arthur's flashbacks of past family reunions being set in July each year and his negative experiences regarding his cousin Mo: he is 5 years old when his cousin puts him on the spinning lawn sprinkler, 6 years old when she does judo on him, and 7 years old when she dressed him up as a baby.
While "Arthur's Cousin Catastrophe" aired in 1996 during the show's first season, the calendar transition during Arthur's flashback of reunions taking place in July has the first of the month on a Wednesday. The first of July in 1996 was on a Monday.