The two meanings

“Every second Tuesday” can mean fortnightly — a Tuesday, then two weeks later, then two weeks after that. Or it can mean the second Tuesday of each month, which is monthly and lands on a different day-of-month each time.

In iCalendar terms the first is FREQ=WEEKLY;INTERVAL=2;BYDAY=TU and the second is FREQ=MONTHLY;BYDAY=2TU. Try both in the expander: over six months the fortnightly rule gives you about thirteen dates and the monthly one gives you six, and they only occasionally coincide.

Decide which you mean before you configure anything. It is much easier than correcting posters.

Fortnightly has a subtlety of its own

Fortnightly rules are anchored to their start date, so “every other Tuesday” starting the 1st and the same rule starting the 8th produce two completely different sets of dates. There is no universal notion of which weeks are the “on” weeks — it depends entirely on where you started counting.

The formal version of this is WKST, which declares which day a week starts on, and it changes the answer for interval rules that span a week boundary. A calendar that ignores it will disagree with Google about the same rule. It is genuinely obscure and it is genuinely the reason two calendars sometimes show different fortnights.

Why the monthly version breaks plugins

“The second Tuesday of the month” requires the plugin to understand an ordinal attached to a weekday. A surprising number do not — they read the frequency and the interval, ignore the ordinal, and produce “the same date each month”. No error, no warning, just wrong dates.

The dates look plausible, which is the problem. Nobody checks a calendar that shows a date every month. It is discovered when somebody turns up on the wrong night.

The expander above handles ordinals, including negatives — -1FR for the last Friday, -2SU for the second-to-last Sunday. Use it as a reference against whatever you are running.

The five-Tuesday problem

Some months have five Tuesdays. “The second Tuesday” is unaffected, but “the last Tuesday” and “the fourth Tuesday” are different dates in those months, and a lot of venues discover they meant one and configured the other. Expand a year and look at the months with five: if the dates surprise you, you meant the other rule.