PIE base ‘to shine’ (and in many derivatives, ‘sky, heaven, god’). See also *bhā-¹ ‘to shine’, root of epiphany.
Other derivatives include Tuesday, deity, jovial, Jupiter, diary, dismal, journey, and psychedelic. Also Latin deus ‘god’.
[Pokorny *dei-1, dei̯ə-, dī-, di̯ā-, pp. 183–187, ‘to shine, glitter; day, sun; god, deity’.]