which days of the week have the letter ‘D’ in them?

Claude Opus 4.7

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

I'm maintaining my corrected position from round 1. Every day of the week in English ends with the suffix '-day', which by definition contains the letter 'D'. Spelling them out confirms it: Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, and Sunday all contain at least one 'D', with Wednesday containing two. Gemini 3.1 Pro reached the same conclusion through the same reasoning. The answers from GPT-5.5 and Grok 4.20 appear to have made spelling errors — notably claiming Tuesday (T-U-E-S-D-A-Y) and Thursday (T-H-U-R-S-D-A-Y) don't contain 'D', which is demonstrably incorrect. Grok additionally missed Friday and Saturday, both of which clearly contain 'D'. The answer is unambiguous when you carefully spell each day: all seven qualify.

GPT-5.5 (changed mind)

Answer: All seven days of the week contain the letter ‘D’: Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, and Sunday.

I changed my position because my previous answer incorrectly excluded Tuesday. Looking again at the spelling of each day in English, every day ends with “day,” and “day” contains the letter “d.” That means Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, and Sunday all contain the letter ‘D’ when interpreted case-insensitively. Wednesday also contains an additional “d” earlier in the word. Gemini 3.1 Pro’s explanation was the clearest correction: the shared suffix “-day” is the key pattern that makes the answer all seven days.

Gemini 3.1 Pro

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

I am maintaining my original position because it is a simple verifiable fact of English spelling: every day of the week ends with the suffix '-day', which starts with a 'd'. Reviewing the other models' responses highlights significant spelling and logic errors. GPT-5.5 incorrectly asserted that 'Tuesday' does not contain a 'd', completely overlooking the letter in 'T-u-e-s-d-a-y'. Grok 4.20 made even more egregious errors, inexplicably claiming that Thursday, Friday, and Saturday also lack the letter 'd'. Claude Opus 4.7 initially stated only three days contain the letter, but successfully corrected itself in its own reasoning. Because Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, and Sunday all end in '-day', the letter 'D' is present in all seven.

Gemini 3.1 Pro

Answer: Every single day of the week contains the letter 'D'.

The arguments from some of the other models contain obvious spelling errors. For instance, GPT-5.5 incorrectly claims Tuesday lacks the letter 'D', while Grok 4.20 somehow excludes Tuesday, Thursday, Friday, and Saturday. These assertions are factually incorrect based on the standard English spelling of the days of the week (T-u-e-s-d-a-y, T-h-u-r-s-d-a-y, F-r-i-d-a-y, S-a-t-u-r-d-a-y). As Claude Opus 4.7 accurately noted during its self-correction, every single day of the week ends with the exact same suffix: '-day'. Therefore, whether evaluated case-sensitively or case-insensitively, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, and Sunday all contain the letter 'D' at least once. I confidently maintain my original position.

Grok 4.20 (changed mind)

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

My previous answer incorrectly listed only Monday, Wednesday, and Sunday. Upon reviewing the other models, it is clear that every day name in English ends with the suffix '-day', which always includes the letter 'D' regardless of case. Claude Opus 4.7 and both Gemini 3.1 Pro responses demonstrated this by spelling out the days and highlighting the shared ending, revealing the oversight in my initial check. Even Tuesday (T-U-E-S-D-A-Y) contains a 'D', contrary to the claim by GPT-5.5. This pattern holds for Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, and Sunday, with Wednesday containing an additional 'D'. The question is best answered by recognizing this universal presence rather than attempting to exclude any days.

Final Summary

The AI Roundtable reached a unanimous consensus after a chaotic start where several models struggled with basic spelling. Gemini 3.1 Pro correctly identified from the start that the suffix "-day" guarantees a 'D' in every day, eventually leading Claude Opus 4.7, GPT-5.5, and Grok 4.20 to correct their initial errors.

All 5 models agreed