In [2]: pd.DatetimeIndex(['1976-11-01', '1976-12-01', '1977-01-03'], freq='BMS')
Out[2]: DatetimeIndex(['1976-11-01', '1976-12-01', '1977-01-03'], dtype='datetime64[s]', freq='BMS')

In [3]: arr = pd.DatetimeIndex(['1976-11-01', '1976-12-01', '1977-01-03'], freq='BMS')

In [4]: arr.is_year_start
Out[4]: array([False, False,  True])

In [5]: [x.is_year_start for x in arr]
Out[5]: [False, False, False]

import pytest
import hypothesis.strategies as st
from hypothesis import given
import pandas as pd
from datetime import datetime


@pytest.mark.parametrize('freq', [
    'MS',
    'BMS',
])
@given(dt = st.datetimes(min_value=datetime(1960, 1, 1), max_value=datetime(1980, 1, 1)))
def test_me(freq, dt):
    d = pd.date_range(dt, periods=3, freq=freq)
    result = [x for x in d.is_year_start]
    expected = [x.is_year_start for x in d]
    assert result == expected

@given(
    dt = st.datetimes(min_value=datetime(1960, 1, 1), max_value=datetime(1980, 1, 1)),
    n = st.integers(min_value=1, max_value=10),
    freq = st.sampled_from(['BMS'])
)
def test_me(freq, dt, n):
    freq = f'{n}{freq}'
    d = pd.date_range(dt, periods=3, freq=freq)
    result = [x for x in d.is_year_start]
    expected = []
    for x in d:
        if x.is_year_start:
            expected.append(True)
        else:
            if x.day_of_week == 0 and (x - pd.Timedelta(days=1)).is_year_start:
                expected.append(True)
            elif x.day_of_week == 0 and (x - pd.Timedelta(days=2)).is_year_start:
                expected.append(True)
            else:
                expected.append(False)
    assert result == expected