Both Vaishnavas and Smarthas observe this fast in place of a separate Ekadashi fast.
Ekadashi Timeline
🌅 Previous Sunrise
Sep 13
06:09:45
🌘 Ekadashi Begins
Sep 13
22:30:22
🌅 Sunrise
Sep 14
06:09:45
🌇 Sunset
Sep 14
18:20:41
🌒 Ekadashi Ends
Sep 14
20:41:37
🌅 Next Sunrise
Sep 15
06:09:45
Dvadashi Timeline
🌅 Previous Sunrise
Sep 13
06:09:45
🌅 Sunrise
Sep 14
06:09:45
🌇 Sunset
Sep 14
18:20:41
🌘 Dvadashi Begins
Sep 14
20:41:37
🌅 Next Sunrise
Sep 15
06:09:45
🌒 Dvadashi Ends
Sep 15
18:12:24
Shravana Nakshatra Timeline
🌅 Previous Sunrise
Sep 13
06:09:45
🌅 Sunrise
Sep 14
06:09:45
🌇 Sunset
Sep 14
18:20:41
⭐ Nakshatra Begins
Sep 14
20:32:24
🌅 Next Sunrise
Sep 15
06:09:45
⭐ Nakshatra Ends
Sep 15
18:48:58
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.