A solemn mass is the most elaborate and formal of the regular Sunday masses.
The principal mass on Sunday (11:15am) is a solemn mass. Also, major holy days will include at least one solemn mass.
A solemn mass at the Cathedral of Our Lady of Walsingham will always include:
An acolyte (or other sacred minister in the subdiaconal role) in addition to celebrant and deacon.
A thurifer (in addition to crucifer and torch bearers) usually with boat boy, MC and two extra servers.
Sung Ordinary and Propers of the Mass, either by the choir alone or together with the congregation.