Look at these four dates in the year 2026: 4 April, 6 June, 8 August and 9 September. Three of them fall on the same day of the week. Which one is the odd date out?

You do not need to know any actual weekday — only the gaps. From 4 April to 4 June is 30 + 31 = 61 days, so 4 April to 6 June is 63 days, and $63=9\times 7$, a whole number of weeks. From 6 June to 6 August is 30 + 31 = 61 days, so 6 June to 8 August is again 63 days. Those three dates therefore share a weekday. But 8 August to 8 September is 31 days, so 8 August to 9 September is 32 days, which is 4 weeks and 4 days over — a different weekday. The traps: all four dates have the same day number as their month number, so they all look alike at a glance and any of them can seem 'the different one'. 8 August is the only one of the four in a month of 31 days; 4 April is the only one in the first half of the year; and 6 June sits neatly between the other two. None of those surface differences is the rule being tested.