Asap i say! Go on a week day if you must. The later it gets in june, the more crowds you will see due to the schools getting out for summer vacation. I believe summer nightastic starts in june as well but i don't know the specific date.
"Ladies and gentlemen, boys and girls, Disneyland proudly presents our spectacular festival pageant of nighttime magic and imagination in thousands of sparkling lights and electro-synthe-magnetic musical sounds. The Main Street Electrical Parade!"