Reindex daily series s to only every 3rd day starting from its first date, filling in any date that isn't already present with the most recent prior value (forward fill).

Input

2020-01-01    0.0
2020-01-02    1.0
2020-01-03    2.0
2020-01-04    3.0
2020-01-05    4.0
2020-01-06    5.0
2020-01-07    6.0
2020-01-08    7.0
2020-01-09    8.0
2020-01-10    9.0
Freq: D, dtype: float64

Output

2020-01-01    0.0
2020-01-04    3.0
2020-01-07    6.0
2020-01-10    9.0
Freq: 3D, dtype: float64