"Sundays" is a homophone of "sundaes". Sundays is the plural of Sunday, which is a day in the week. Sundaes is the plural of sundae which is a type of dessert involving ice cream. A sundae typically has dairy as a main ingredient. A Dairy Queen is a fast food chain that serves ice cream. Tom is saying that they are not a Dairy Queen, but they have "great Sundays" meaning their Sundays are fun filled, but "great Sundays" sounds like "great sundaes", so "great ice cream". This joke is playing on the word Sundays and its homophone sundaes. 
We Are Not Dairy Queen … But We Have Great Sundays