Is this grammatically correct?
The day of rest became a day of violence when a bomb exploded in a Belfast cafe.
Or is this better?
A day of rest became a day of violence when a bomb exploded in a Belfast cafe.
"Day of rest" refers to Sunday.
They're both grammatically correct, but they mean different things. "The day of rest" is synonymous with "the Sabbath" ie (in this case) Sunday. "A day of rest" could be any day in which the people involved were not working, for any reason. It might, for instance, be the other day in the weekend (Saturday for Christians) or some other general holiday such as Easter Monday or a saint's day. If Sunday is the intention, then the first is both concise and stylistically tidy.