Siddha Tithis are beneficial and are considered highly
auspicious. All kinds of work undertaken on such a day will result in success.
Here is how to calculate Siddha Tithi.
Tithi 3, 8, 13 during both paksha (fortnight) falling on
Tuesday is Siddha Tithi
Tithi 2, 7, 12 during both paksha falling on Wednesday is
good
Tithi 5, 10, 15 during both paksha falling on Thursday is
good
Tithi 1, 6, 11 during both paksha falling on Friday is good