Many features in Interesting Stuff will be stable and therefore presented on a specific weekday, possibly Friday's Foreign Phrase for instance. In order to provide some flexibility, less certain content can be alternated from week to week. This is done by designating weeks as A, B, or C as follows: Weeks alternate between A and B, except for the first 7 days at the beginning of the month which interrupt this cycle with the designation C. The calendar below illustrates the resulting pattern.

Each week is also assigned a number to uniquely identify it. For instance, weeks designated as C are assigned the numerical month. There is only one C3 Tuesday, and there is always one C3 Tuesday, just to pick a random example, namely the first Tuesday of March. Weeks designated A and B run Monday through Sunday (apart from partial weeks like B0) though they can be interrupted at month boundaries. There are always at least 20 weeks designated A, and likewise for B. There is exactly one A3 Tuesday, to pick another example, and exactly one B19 Friday. Although year boundaries complicate the situation per ISO 8601, these can be adjusted as desired.

There are ${\displaystyle 12\times 7}$ days of the year that are designated C, or about 23%. The remaining days are divided more or less evenly between A and B, although slightly favored toward the starting designation B (39% to 38%).

Consider a single weekday such as Thursdays. A stable feature will be presented 100% of the time, on any designated day A, B, or C. If a feature is to be displayed just at the beginning of the month with C and a more certain feature with the combination of A and B, their ratios will be about 23% to 77%. A feature selected for B would show 39% of the time versus A and C together at 61%.

Today's date of July 11 is listed above as B10 Friday. From the main page —and only from there— the week designation is determined by the following code:

{{#ifexpr:{{CURRENTDAY}}>7|{{#ifexpr:({{CURRENTWEEK}}-{{CURRENTMONTH}}-1)mod2|A|B}}|C}}

The weekday (7 for Sunday) and week designation (A, B, or C) are used to select a feature for the current day, as determined by Wiktionary:Main Page/interesting. Because the features are archived and cycled over the year, as with Word of the Day, the week number determined by:

{{#ifexpr:{{CURRENTDAY}}>7|{{#expr:ceil(({{CURRENTWEEK}}-{{CURRENTMONTH}}-1)/2)}}|{{#expr:{{CURRENTMONTH}}}}}}

provides the specific article, for instance phrasebook/B5 or phrasebook/B19 or similar. Note that the offset of -1 italicized in both samples above can be adjusted to make the year start and end on the desired week designation.