Jai Shri Krishna

Here is an overview as per my limited understanding. Please excuse any errors and kindly help to correct the same

ekadashi = ek+dash = 1+10 = 11 .... Agyaaras.

This day occours twice in every given month, towards beginning (Sud) of the month and towards the end (Vad) of the month. (according to the hindu calendar and according to the pushtimargiya tippani ji - Vaishnavs refer to Tippaniji as there can be conflicting events in the generic calendar and also the generic calendar does not give all the events related to pushtimarg)

The process of keeping the 11 indriyas (senses) in control and is usually the process of abstaining from certain foodstuff such as grains, etc which contain rajas and tamas qualities/guns and thus keeping focused in one-pointed way towards spiritual matters (i.e. in thakorji). The foodstuff one is allowed to partake is sattvick in nature, such as fruits, roots,  nuts, etc

The reasons for keeping the discipline of doing the ekadashis are described in various mahapurans (bhavishya-uttar puran, bhramaand puran, mahabharat - such as discussions betweeen shri krishna and yudhisthir, etc) as per maryada.

In our pushtimarg the bhaav behind each ekadashi is different and the bhaavs are directly linked with shri thakorji. Shri Mahaprabhuji has given agya that vaishnavs should do the 24 ekadashis (2 in each month 12 months in a year - and 2 extra in case where there is adhik month - an extra month) and 4 jayantis (shri nrusingh jayanti, shri vaman jayanti, shri ram navami and shri krishna janamashtami)

Hope this helps

Jai Shri Krishna
Jagdish