Try dict comprehension
:
>>> {f'Day {i}': Counter(sublist) for i, sublist in enumerate(l, start=1)}
{'Day 1': Counter({'a': 3, 'b': 1, 'c': 1}),
'Day 2': Counter({'a': 2, 'b': 2, 'c': 1}),
'Day 3': Counter({'a': 1, 'b': 2, 'c': 2})}
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