From 8-year old Hilda’s diary:

I must stay home today and do my homework because I am far behind the rest of the class. Grandfather says that it is not because I was stupid but because I was sick. My homework for today is to think about what is rare about a day in June. In class today, Miss Hare made us take our pens and write in our blank books, “And what is so rare as a day in June.” She said it is the first line of a beautiful poem by an American poet whose name is James Russell Lowell and she made us write down his name too so we would remember it but she said the poem was very long and for now the first line was enough. We must think about the first line over the weekend and on Monday morning we must be able to tell her why a day in June is beautiful and what the word rare means.

Apparently many people have asked the same question. A google search for the meaning of “And what is so rare as a day in June” brought up 361,000,000 results! The poem is from the Prelude of a much longer poem called “The Vision of Sir Launfal.”