Both Vaishnavas and Smarthas observe this fast in place of a separate Ekadashi fast.
Ekadashi Timeline
🌅 Previous Sunrise
Sep 20
06:09:47
🌘 Ekadashi Begins
Sep 20
10:08:21
🌅 Sunrise
Sep 21
06:09:48
🌒 Ekadashi Ends
Sep 21
11:11:02
🌇 Sunset
Sep 21
18:15:30
🌅 Next Sunrise
Sep 22
06:09:48
Dvadashi Timeline
🌅 Previous Sunrise
Sep 20
06:09:47
🌅 Sunrise
Sep 21
06:09:48
🌘 Dvadashi Begins
Sep 21
11:11:02
🌇 Sunset
Sep 21
18:15:30
🌅 Next Sunrise
Sep 22
06:09:48
🌒 Dvadashi Ends
Sep 22
12:42:36
Shravana Nakshatra Timeline
🌅 Previous Sunrise
Sep 20
06:09:47
⭐ Nakshatra Begins
Sep 20
23:01:13
🌅 Sunrise
Sep 21
06:09:48
🌇 Sunset
Sep 21
18:15:30
⭐ Nakshatra Ends
Sep 22
00:55:13
🌅 Next Sunrise
Sep 22
06:09:48
About
Vishnu Shrinkhala Yoga
Vishnu Shrinkhala Yoga (Vishnu's Chain) occurs when Ekadashi, Dvadashi, and Shravana Nakshatra all overlap together on the first of Shravana Dvadashi's two candidate days, with Shravana Nakshatra directly touching both tithis -- overlapping with Dvadashi alone does not form this yoga.
This is not an annual occurrence: some years the alignment falls into place, other years it does not, since it depends on where Shravana Nakshatra happens to fall relative to that year's Ekadashi-Dvadashi boundary. When it occurs, the Shravana Dvadashi fast is observed on the first day; both Vaishnavas and Smarthas fast on this day in place of a separate Ekadashi fast.
Like most panchang determinations, this has been interpreted somewhat differently across sampradayas over the centuries -- please follow the convention observed by your own family or tradition where it differs from what is shown here.