Notes

CREATE TABLE RATES (Date DATE, Rates FLOAT)
CREATE TABLE SELLS (Date DATE, Sells INT)

INSERT INTO RATES (Date, Rates)
VALUES ('2019-12-26', '61.4676'),
('2019-12-27', '61.4567'),
('2019-12-28', '61.5678'),
('2019-12-31', '61.4235')

INSERT INTO SELLS (Date, Sells)
VALUES ('2019-12-25', '0'),
('2019-12-26','0'),
('2019-12-27', '0'),
('2019-12-28', '0'),
('2019-12-29', '100'),
('2019-12-31', '0')


SELECT Sells.Date,Rates , Sells FROM RATES
FULL JOIN SELLs
ON Rates.date = Sells.date
GROUP BY Sells.Date, Rates, Sells

;WITH trn as (
SELECT *
,row_number()over(order by Date) rn
FROM (SELECT SELLS.DATE, Rates FROM RATES
FULL JOIN SELLS
ON RATES.date = SELLS.date) AS Rates2
WHERE Rates is not null
),
data as(
select t1.Date,t1.Rates
,t2.Date nextdate
from trn t1
left join trn t2 on t1.rn=(t2.rn-1)
)
select t1.Date,coalesce(t1.Rates,t2.Rates) Rates
,t2.Date from_date
from Rates2 t1
left join data t2 on t2.Date<t1.Date and t2.nextdate>t1.Date