Sri Vasu vadhyar,

In Paranthaman panchangam for vyaya varusham, under do
and don't do list I find:
dos On Janma, anujanma, trijanma nakshatram:
homam, business, wearing jewellery, shanti, vivaham,

don't on Janma, anujanma, trijanma nakshatram:
sraddham, travel, vaidyam, marriage for male,
rutu sangamam and seemantham.

Q1. What are janma, anujanma and trijanma nakshatrams
a person?
Q2. Shashtiapta poorthi, sadabhishekam are observed on
the janma nakshatram (birth month/birth star). Usually
on apta poorthi wedding is performed between the
(husband and wife). How can a marriage be performed on
janma nakshatram day?


***Everybody know 'Janma Star' very well.
Anujanma means the 10th Star and the Thrijanma is the 19th star.
For example for a person of Janma Star 'Aswini'- Anujanma star is 'Makam' and
Thrijanama star is 'Moolam'.
This also given in sets in the 23rd page of " Paranthaman " Panjangam.

I could not understand your Question number 2:
If you are mentioning the " Shastiyaptha Poorthi " as marriage.
You should know, that " Shaanthi on Janma Nakshathram " is the main thing (main
ritual) on that day and not the " Thirumangalya dhaaranam " . Thirumangalyam is a
added ritual to Shastiyaptha poorthi. Also, one should not think it is same as
the wedding. Wedding is having full of rituals instructed by 'Sastram' which is
a must. But, 'Shastiyaptha poorthi' is not a 'Samskaaram', mere (Kaamya)
'Shaanthi - Karma' and hence it is not a must, optional.