"The Lost Summer" is a fandom term for the period between the end of Daria Morgendorffer's sophomore year at Lawndale High and the start of her junior year. Whereas Daria's junior year/senior year summer is shown in full in "Is It Fall Yet?", her first summer in Lawndale is never mentioned on-screen at all.
It is popularly believed that "Road Worrier" (and possibly "The Teachings of Don Jake") took place then, as "The Daria Diaries" places an Alternapalooza festival in August, but this is never confirmed in the episodes.
It is rare to see fanfic set in this period, which is another reason why it is called the "lost" summer. Some exceptions are: