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 other models, it is clear that my initial analysis missed a fundamental linguistic pattern: every day of the week in English ends with the suffix "-day." This suffix inherently includes the letter 'd,' which means all seven days—Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, and Sunday—contain at least one 'd.' While my previous answer correctly identified four days with explicit 'd's (Monday, Wednesday, Thursday, and Saturday), it overlooked the implicit 'd' in the "-day" suffix present in every day's name. This oversight was pointed out by multiple models, including Claude Opus 4.8, GPT-5.5, Gemini 3.1 Pro Preview, and Kimi K2.6, whose arguments highlight the universal presence of the 'd' in the suffix. Furthermore, Wednesday contains an additional 'd' in its spelling ("Wed-nes-day"), reinforcing that all days meet the criterion. There is no exception to this rule in standard English nomenclature for the days of the week. Therefore, the correct count is unambiguously seven.

GPT-5.5

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

I maintain my previous position because the standard English day names are Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, and Sunday, and each contains the letter "d". The simplest reason is that every one of these names contains the suffix "day," which begins with "d." So Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, and Sunday all include at least one "d." Some contrary answers overlooked the "d" in names like Tuesday, Friday, and Sunday by apparently focusing on earlier syllables rather than the full spelling. The ambiguity point is theoretically true if no language is specified, but in this context the ordinary interpretation is English day names, under which the answer is clearly 7.

Claude Opus 4.8

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

I maintain my corrected answer of seven. Every day name in English ends with the suffix '-day', which guarantees the letter 'd' is present in each one: Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, and Sunday. The claim by Grok 4.3 that Tuesday, Friday, and Sunday lack a 'd' is simply incorrect: Tues-DAY, Fri-DAY, and Sun-DAY all contain 'd'. Notably, Grok also misspelled Saturday as 'Sadurday' to find its 'd', when the actual 'd' is in 'Satur-DAY'. The 'four' answers fail to account for the universal '-day' ending. DeepSeek's appeal to ambiguity is unpersuasive. The question clearly refers to the standard English names for the days of the week — the only reasonable default interpretation — and under that interpretation the answer is unambiguous. Declining to answer overcomplicates a settled question. The key insight, which GPT-5.5, Gemini 3.1 Pro Preview, and Kimi K2.6 all share, is that the '-day' suffix appears in every single name. Therefore the correct count is seven.

DeepSeek V4 Pro (changed mind)

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

Upon reviewing the other models' answers, I discovered an error in my initial reasoning. I had stated that only Wednesday, Thursday, Saturday, and Sunday contain the letter 'd', overlooking the 'd' in Monday, Tuesday, and Friday. The correct observation is that every standard English day name — Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, and Sunday — ends with the suffix '-day', which guarantees the presence of the letter 'd' in each name. This simple linguistic fact was clearly and convincingly pointed out by the majority of the respondents, and it resolves the ambiguity I initially cited. The question's reference to 'days of the week' in a context discussed by all models in English makes the frame of reference clear. Therefore, my previous answer was incorrect, and I now agree that the answer is seven.

Gemini 3.1 Pro Preview

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

I am maintaining my original position because the English names for the days of the week—Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, and Sunday—unquestionably all end with the word 'day'. Therefore, each of the seven days contains at least one 'd'. Reviewing the other models' responses only reinforces this conclusion, as those arguing for a different count made glaring spelling errors or linguistic hallucinations. For instance, Grok 4.3 inexplicably claimed Tuesday, Friday, and Sunday do not contain a 'd', and even misspelled Saturday as 'Sadurday'. DeepSeek V4 Pro similarly hallucinated which days contain the letter. Meanwhile, GPT-5.5, Kimi K2.6, and Claude Opus 4.8 (after a brief self-correction) all independently verified that every single day of the week contains the letter 'd'. The underlying fact remains straightforward: because every day ends in '-day', the correct total is seven.

Kimi K2.6

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

I maintain that the answer is seven. A simple spelling check of every English day name confirms this: Monday (Mon-d-ay), Tuesday (Tues-d-ay), Wednesday (Wed-nes-d-ay), Thursday (Thurs-d-ay), Friday (Fri-d-ay), Saturday (Satur-d-ay), and Sunday (Sun-d-ay) each contain the letter "d." Some models incorrectly excluded certain days—Grok 4.3 omitted Tuesday, Friday, and Sunday, while DeepSeek V4 Pro omitted Monday, Tuesday, and Friday. Both appear to have misread the spelling of the day names. However, Claude Opus 4.8, GPT-5.5, and Gemini 3.1 Pro Preview all independently verified that every day ends in the "-day" suffix, which guarantees the presence of "d" in all seven names. Because the factual evidence is unambiguous and multiple independent checks confirm the total is seven, I see no reason to change my original position.

Final Summary

The AI Roundtable ultimately reached a unanimous consensus that all seven days of the week contain the letter 'd'. GPT-5.5, Gemini 3.1 Pro Preview, and Kimi K2.6 noted right away that every day ends in the suffix '-day', while Claude Opus 4.8 quickly self-corrected to the same conclusion. Grok 4.3 and DeepSeek V4 Pro initially faltered with spelling blunders and claims of ambiguity, but both models revised their positions in the final round to agree on seven.

All 6 models agreed