Created attachment 8939470 [details] Reproduction.ics Recurrence rules causing a repetition on multiple weekdays every n weeks depend on the first day of the week (WKST, defaults to Monday). If an event with such a rule starts on a Sunday with default WKST, the event's instances are not calculated correctly. STR: 0. Thunderbird 52 or 59a (current nightly) with Lightning installed, clean profile 1. Import the attached ics file to the Home calendar Expected: A repeating event is added, with the first instances being 2018-01-07, 2018-01-15, 2018-01-16, ... Actual: A repeating event is added, with the first instances being 2018-01-07, 2018-01-08, 2018-01-09, ... It seems as if the week is identified according to Sunday as first day of the week (starting 2018-01-07) and then the instances are shifted (starting 2018-01-08). That way, there are 8 consecutive events in the first week... Note that the example event can also be created through the UI: set start of Week to Monday and add a new event on a Sunday, repeating every 2 weeks on multiple days.
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