let's image the background of the poem: the poet worked in a foreign land alone without his family, it is no doubting that he would miss his family, especially in the Festival when he should be reunited with his family. and so he wrote the poem.

独在异乡为异客，每逢佳节倍思亲 As a stranger in a strange land, he misses his family the most during holidays/festive times/festive days.

Alone in a foreign land as an example, during the Festival festive "