The distribution of Easter dates

from collections import defaultdict
import matplotlib.pyplot as plt

# The Gregorian Easter dates repeat every 5.7 million years.
PERIOD = 5_700_000

def get_easter_date(year):
    """Return the day and month of Easter in a requested year."""

    # Use the anonymous algorithm, Anon., "To find Easter", Nature, p.487
    # (20 April 1876) as described by Wikipedia:
    # https://en.wikipedia.org/wiki/Date_of_Easter#Anonymous_Gregorian_algorithm
    a = year % 19
    b, c = divmod(year, 100)
    d, e = divmod(b, 4)
    f = (b + 8) // 25
    g = (b - f + 1) // 3
    h = (19*a + b - d - g + 15) % 30
    i, k = divmod(c, 4)
    l = (32 + 2*(e + i) - h - k) % 7
    m = (a + 11*h + 22*l) // 451
    month, day = divmod(h + l - 7*m + 114, 31)
    day += 1
    return day, month

# Count the easter dates across the whole cycle.
counts = defaultdict(int)
for year in range(1, PERIOD+1):
    day, month = get_easter_date(year)
    counts[(month, day)] += 1

# Sort the dates and get the percentages of Easters by date.
dates = sorted(counts)
percentages = dict((date, counts[date] / PERIOD * 100) for date in dates)

# Plot a bar chart
fig, ax = plt.subplots(figsize=(10, 5))
idx = range(len(counts))
ax.bar(idx, percentages.values())
ax.set_xticks(idx)
ax.set_xticklabels([str(day) for month, day in dates])
ax.set_ylabel('% of Easters')
ax.text(0, -0.4, 'March')
ax.text(10, -0.4, 'April')
plt.show()