Twas is a contraction of the phrase 'it was' and is commonly used in the opening line of a yuletide poem. It is an archaic form of writing that is still used today to evoke a sense of nostalgia and tradition. Yule tide poems are typically written around Christmas time and often tell stories of the holiday season. The opening line of a yuletide poem usually begins with 'Twas' followed by a description of the setting or characters.