If you read the fine print on Disneyland's website, you'll see that it's typically on Monday, Wednesday, Friday, and Saturday.:
"* To enter Mickey's Toontown Morning Madness, each Guest must have a valid 2-Day or greater Disneyland Resort Park Hopper souvenir ticket and one Mickey's Toontown Morning Madness voucher from the Walt Disney Travel Company. Entitlement available for one morning throughout the length of your ticket on Monday, Wednesday, Friday or Saturday. Days and times of operation and all other elements including but not limited to operation of attractions, entertainment, stores and restaurants, and appearance of Characters may vary and are subject to change without notice. Subject to capacity and other restrictions."