Tuples (datetime, description) all are sorted by the date except one entry (2021-03-09) which is out of order:

        Expected order:             Actual order:
        2021-03-04 Event E          2021-03-04 Event E
        2021-03-07 Event B          2021-03-07 Event B
        2021-03-08 Event C          2021-03-08 Event C
        2021-03-09 Event A          2021-03-11 Event D
        2021-03-11 Event D          2021-03-09 Event A

In REPL it can be replicated by pasting the following code:

import heapq
from datetime import datetime

event_a = (datetime.strptime('2021-03-09', '%Y-%m-%d'), "Event A")
event_b = (datetime.strptime('2021-03-07', '%Y-%m-%d'), "Event B")
event_c = (datetime.strptime('2021-03-08', '%Y-%m-%d'), "Event C")
event_d = (datetime.strptime('2021-03-11', '%Y-%m-%d'), "Event D")
event_e = (datetime.strptime('2021-03-04', '%Y-%m-%d'), "Event E")

events = []
heapq.heappush(events, event_a)
heapq.heappush(events, event_b)
heapq.heappush(events, event_c)
heapq.heappush(events, event_d)
heapq.heappush(events, event_e)

expected_list = [event_e, event_b, event_c, event_a, event_d]

assert events == expected_list