Some common synonyms of formal are ceremonial, ceremonious, and conventional. While all these words mean "marked by attention to or adhering strictly to prescribed forms," formal applies both to things prescribed by and to persons obedient to custom and may suggest stiff, restrained, or old-fashioned behavior.

// a formal report

// the headmaster's formal manner

While in some cases nearly identical to formal, ceremonious applies to persons given to ceremony or to acts attended by ceremony.

// made his ceremonious entrance

Ceremonial and ceremonious both imply strict attention to what is prescribed by custom or by ritual, but ceremonial applies to things that are associated with ceremonies.

// a ceremonial offering