Vaikunta Ekadashi and Shivarathri are very popular Hindu festivals.
If you observe carefully both Vaikunta Ekadashi which happens in Dhanu Masa, which is one masa before Makara masa and Shivarathri which happens in Magha masa, when Sun is in Makara Rashi.
As of now they are celebrated on the Shukla Ekadashi of Dhanu Masa and Magha Krishna Chaturdashi respectively.
What is the source of this? Which ancient authentic scripture gives the date for the celebration for these? Is there any seasonal relation to these festivals?
Also is there any relation between these festivals and Uttarayana?