comparing with today and yesterday record in sql query

1declare @bgn_dt date = '2017-12-15' --set by OP
2    , @end_dt date = '2017-12-22' --set by OP
3    , @lag_dt date;
4
5set @lag_dt = (select max(MyDate) from #myTable where MyDate < @bgn_dt) --get the "yesterday" that the @bgn_dt will need
6
7select a.MyDate
8, a.SalesTotal
9, format(((1.0 * a.SalesTotal) / a.SalesTotalPrevDay) - 1, '0%') as SalesTotalChange
10from (
11    select t.MyDate
12    , t.SalesTotal
13    , lag(t.SalesTotal, 1, NULL) over (/*partition by (if needed)*/ order by t.MyDate asc) as SalesTotalPrevDay
14    from #myTable as t
15    where 1=1
16    and t.MyDate between @lag_dt and @end_dt
17    ) as a
18where 1=1
19and a.MyDate >= @bgn_dt
20