Generally it's seen that ekadashi or any other date for that purpose falls on two days. Normally Smartha observe say,ekadashi one day earlier than Vaishnavas. Most calendars only show Smartha dates but some show both. For instance my local calendar has two days for Janamashtmi,11 august for Smarthas and 12 august for Vaishnavas . This distinction is very acutely observed in North India even though Hindus there are not much sectarian other wise.
Why is it so?