Every century has its own anchor day — the day of the week that century's Doomsday method starts counting from. These four anchors loop in a fixed order and repeat every 400 years:

Once you know your century's anchor, here's how you turn any year into its Doomsday, using the "Odd+11" method:

• Take the last two digits of the year
• Odd? Add 11
• Divide by 2
• Odd? Add 11
• Divide the result by 7 and keep the remainder (that is, 'mod-7')
• Subtract that remainder from 7 — that's your complement
• Count that many days forward from your century's anchor day — that lands you on the year's Doomsday

Clunky the first few times, but soon it'll take just a few seconds.

Once you've got a year's Doomsday, you've got a free anchor point in every single month — because these dates always fall on the Doomsday:

• 3 Jan (4 Jan in a leap year)
• The last day of February (28th, or 29th in a leap year)
• 14 Mar (Pi day)
• 4/4, 6/6, 8/8, 10/10, 12/12 (the even months, doubled up)
• 9/5 and 5/9 ("working 9-to-5 at the 7-11")
• 7/11 and 11/7

A few memorable holiday dates also fall on the Doomsday:

• 4 July — US Independence Day
• 31 October — Halloween
• 26 December — Boxing Day

To get any other date find the nearest key date to your target, then count forward or backward through the week — wrapping past Saturday back to Sunday (or past Sunday back to Saturday) as needed — the same number of days as the gap between the two dates.

Phew! Let's run through an example.

Step 1 — Find the year's Doomsday

Step 2 — Find the nearest key date

The closest key date to 15 September is 5 September ("working 9-to-5"), which is 10 days before our target — and like every key date, it falls on the year's Doomsday: Saturday.

Step 3 — Count forward

Our target is 10 days after that key date. That's one whole week plus 3 more days... Sat → Sun → Mon → Tue.

That's the whole method, start to finish. Time to make detective.