Nobody ships on a Friday

Two claims in one piece of folklore. (A) Do people avoid releasing software at the end of the week? Measured as Friday's share of Monday-to-Friday releases across 60,178 releases of 980 PyPI projects, 2016 to August 2026, where a flat week is 20 %. (B) Is what ships on a Friday more likely to be a thing that has to be fixed? Measured as the chance that the same project publishes another release within 72 hours, Friday against Monday-to-Thursday.

rule.py, sealed at the 1f916 registry as rule.friday-releases at 2026-09-16 21:12 Rome (epoch 1789585933404), before the first package was fetched; the first rows were on disk at 21:13. Sample: 1,000 projects drawn without replacement from ranks 1–5000 of the public PyPI download ranking snapshot dated 2026-09-01, seed 20260916, the drawn names committed as sample.txt in the same commit as the rule. A release is a distinct non-pre-release version dated by the earliest upload among its files, inside 2016-01-01..2026-09-01 UTC. Claim A, Wilson 95 % on Friday's share of Mon–Fri: survived if the upper bound is below 0.180 (a dip of at least a tenth), killed if the lower bound is above 0.190 (a dip under a twentieth), otherwise inconclusive; and inconclusive regardless if the pooled share and the median per-project share fall on opposite sides of 0.190. Claim B, 95 % interval on the relative risk: survived above 1.10, killed if the whole interval sits inside 0.90–1.10, and — written into the sealed file — a null with fewer than 10,000 eligible Friday releases is inconclusive rather than killed. Negative control, sealed and scored: the same relative risk for Tuesday against Monday/Wednesday/Thursday, which folklore says nothing about; if it lands outside 0.95–1.05 with an interval excluding 1, claim B is void. Positive control: injection at true relative risks 1.05 to 1.30, 400 runs each, reporting the smallest the rule confirms 80 % of the time.

computed after the data — conditional on the world as it came out, so it cannot show a branch was unreachable by design

Claim A inconclusive; claim B void on its own sealed control. A: 72,583 versions fetched, dropped 353 with no files, 8 unparseable, 7,278 pre-releases and 4,766 outside the window, leaving 60,178 releases of 980 projects. Friday holds 18.99 % of the 52,534 Monday-to-Friday releases (95 % CI 18.66–19.33) against 20.00 % for a flat week — a dip of 5.1 %, landing in the one-point band between the sealed bars. The per-project median (505 projects with at least 20 weekday releases) is 18.42 % and agrees on the side, so the tie-break did not fire. Registered and not scored: the weekday histogram is Thu 19.23 %, Wed 17.73 %, Tue 17.58 %, Fri 16.58 %, Mon 16.17 %, Sat 6.74 %, Sun 5.96 % of all seven days; the Friday share by year moves 17.75 % (2016) to 20.27 % (2026) with no monotone trend; the same share under six clock offsets runs 17.43 % at UTC−8 to 23.59 % at UTC+8; the yank rate is 0.89–1.03 % on every weekday and flat. B: 59,955 eligible releases; the 72-hour follow-up rate is 31.89 % on Friday against 45.44 % Monday–Thursday, RR 0.702 (0.681–0.724). The sealed Tuesday control came back RR 1.114 (1.089–1.140), outside the tolerance and excluding 1, so the measurement sees weekday structure unrelated to the claim and the sealed consequence is void. The injection floor says the rule would have caught a true RR of 1.15 in 97.8 % of runs and 1.10 in 3.0 %. After the data, not sealed: Monday is the quietest weekday, not Friday (18.53 % of Mon–Fri against Friday's 18.99 %; Monday's share of Monday-plus-Friday is 49.4 %, CI 48.7–50.1, a tie); applying the sealed bars to each clock gives survived at UTC−8, inconclusive at UTC−5 and UTC+0, killed at UTC+2, UTC+5.5 and UTC+8; and the 72-hour proxy tracks how many working days the window contains (1 working day 35.7 %, 2 days 44.2 %, 3 days 46.6 %), which is what the control caught. At 168 hours — a window holding exactly five working days whichever day it starts on — the Tuesday control comes back inside tolerance at 1.027 and Friday's relative risk is 1.065 (1.046–1.084): above one, and below the bar the sealed rule would have needed. That window was then sealed as its own study on a held-out population the same evening (/experiments/is-a-friday-release-followed-sooner/), where both controls came back clean and the relative risk came back 1.0231 (1.0113-1.0350).

I tested a version of the claim its speakers never asserted.

What is sealed is a claim about publishing a release to a package index. What people say is usually about deploying to production on a Friday afternoon, and no registry records that. A reader who means the second one should treat this as evidence about the first and nothing more.

• A release published on a Friday is more likely to be followed within 72 hours by another release of the same project (the proxy for "it had to be fixed").

  The sealed negative control failed: Tuesday against Monday/Wednesday/Thursday gives RR 1.114 (1.089–1.140), where a measurement of breakage should give 1. The proxy is dominated by how many working days the follow-up window contains — one working day 35.7 %, three 46.6 % — and Friday's window contains one. The claim was not scored; the instrument could not have answered it either way.

  voided 2026-09-16