How many days of the week include d?

Grok 4.3 (changed mind)

Answer: All seven days of the week include the letter 'd'.

Upon reviewing the responses from Gemini 3.5 Flash, GPT-5.5, and Claude Opus 4.8, I recognize a critical oversight in my initial analysis. Each day of the week in English ends with the suffix "-day" (e.g., Monday, Tuesday, etc.), and the word "day" inherently includes the letter 'd'. Initially, I excluded Tuesday and Friday from my count, but this was incorrect. Tuesday (T-U-E-S-**d**-A-Y) and Friday (F-R-I-**d**-A-Y) both contain the letter 'd' as part of their "-day" suffix. Similarly, while I initially miscounted Sunday (S-U-N-**d**-A-Y), it too includes the letter 'd'. The suffix "-day" is consistent across all seven days, ensuring that every day name contains the letter 'd'. This observation, highlighted by the other models, confirms that the correct answer is seven days.

GPT-5.5

Answer: All seven days of the week include the letter “d.”

I maintain my previous position: the answer is 7. The English names of the days are Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, and Sunday. Each one contains the suffix “day,” and since “day” begins with “d,” every day name includes the letter “d.” Gemini 3.5 Flash and Claude Opus 4.8 correctly point out this suffix-based reasoning. Grok 4.3’s answer is internally inconsistent and incorrectly claims that Sunday does not contain “d,” even though “Sunday” is spelled S-u-n-d-a-y.

Gemini 3.5 Flash

Answer: All seven days of the week contain the letter 'd'.

I am maintaining my position because it is a simple linguistic fact that every day of the week in English ends with the suffix '-day', which contains the letter 'd'. Specifically, the days are Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, and Sunday. Looking at the feedback from other models, GPT-5.5 correctly agrees with this position. Claude Opus 4.8 also correctly identifies the suffix '-day' in its reasoning, though its headline stance contains a typo claiming 'Six days'. Grok 4.3's argument is highly flawed and mathematically/orthographically incorrect. It states that 'Sunday does not contain the letter 'd' (it is spelled S-U-N-D-A-Y)', which is a self-contradiction since the letter 'D' is clearly present in 'S-U-N-D-A-Y'. Similarly, Grok claims Tuesday and Friday do not contain the letter 'd', overlooking the '-day' suffix entirely. Therefore, the only logical and factual conclusion remains seven.

Claude Opus 4.8

Answer: All seven days of the week include the letter "d".

I maintain my corrected position from round 1: every day of the week ends in the suffix "-day", which contains the letter "d". This guarantees all seven qualify. Let me verify once more, letter by letter: - Monday: M-O-N-D-A-Y ✓ - Tuesday: T-U-E-S-D-A-Y ✓ - Wednesday: W-E-D-N-E-S-D-A-Y ✓ (two d's) - Thursday: T-H-U-R-S-D-A-Y ✓ - Friday: F-R-I-D-A-Y ✓ - Saturday: S-A-T-U-R-D-A-Y ✓ - Sunday: S-U-N-D-A-Y ✓ Gemini and GPT-5.5 reached the same correct conclusion. Grok's answer of "four" contains clear spelling errors — it wrongly claims Tuesday and Friday lack a 'd', when both end in 'day' (Tues-DAY, Fri-DAY), and it even contradicts itself by mis-spelling Sunday despite writing it correctly as S-U-N-D-A-Y. The "-day" suffix is the decisive structural fact: it is impossible for any English weekday name to lack the letter 'd'. The answer is seven.

Final Summary

The AI Roundtable ultimately reached a unanimous consensus that every single day of the week contains the letter 'd' due to the universal suffix '-day'. While Gemini 3.5 Flash and GPT-5.5 spotted this immediately, Claude Opus 4.8 and Grok 4.3 initially stumbled over bizarre spelling errors—including Grok claiming 'Sunday' lacks a 'd' while literally spelling it 'S-U-N-D-A-Y'—before correcting their positions in the final round.

All 4 models agreed