For those interested in seeing this, a few dates have just been publicized for the upcoming descents - check with the tourist office in Lauterbrunnen for more info. The cows are usually "dressed up" with flowers and bells.
Lauterbrunnen:
24 Sept 21 Alpabzug from the Mäderalp, arriving between 14:30 - 15:00 (along Dorfstrasse)
25 Sept 21 Alpabzug from Winteregg, arriving between 12:00 - 13:00 (along Dorfstrasse)
Wengen/Lauterbrunnen:
27 Sept 21 Alpabzug Wengernalp arriving ca 11:00 in Wengen or if you are in Lauterbrunnen, between 12:00 - 13:00 (along Stocki Strasse in the direction of Kirchenparkplatz parking lot by the church)
https://wengen.swiss/en/summer/product/event/cattle-coming-down-from-wengernalp/
29 Sept 21 Looks like another descent of the mothers and calves, expected in Wengen around 10:30am https://wengen.swiss/en/summer/product/event/cows-and-their-calves-coming-down-from-wengernalp/