Servorum est festus dies hic quem plane hexametro versu non dicere possis.
slave-M-PL-GEN be-3-S holiday-M-S-NOM day-M-S-NOM this-M-S-NOM REL-M-S-ACC plainly hexameter-M-S-ABL verse-M-S-ABL not say-INFIN be able-2-S-SUBJ
This is a slaves' holiday which you simply couldn't say in a hexameter verse.