declare @bgn_dt date = '2017-12-15' --set by OP
    , @end_dt date = '2017-12-22' --set by OP
    , @lag_dt date;

set @lag_dt = (select max(MyDate) from #myTable where MyDate < @bgn_dt) --get the "yesterday" that the @bgn_dt will need

select a.MyDate
, a.SalesTotal
, format(((1.0 * a.SalesTotal) / a.SalesTotalPrevDay) - 1, '0%') as SalesTotalChange
from (
    select t.MyDate
    , t.SalesTotal
    , lag(t.SalesTotal, 1, NULL) over (/*partition by (if needed)*/ order by t.MyDate asc) as SalesTotalPrevDay
    from #myTable as t
    where 1=1
    and t.MyDate between @lag_dt and @end_dt
    ) as a
where 1=1
and a.MyDate >= @bgn_dt