Question: when did the fourth of july become a legal federal holiday
[{'answer': 'In 1870', 'provenance': [{'wikipedia_id': '781729', 'title': 'Independence Day (United States)', 'start_paragraph_id': 14, 'start_character': 12, 'end_paragraph_id': 14, 'end_character': 19, 'bleu_score': 1.0, 'section': 'Section::::Observance.\n'}]}]