Subha Sweekaram means ....
The Karthas family was in a state of Asubham or subham less due to the lose of a valuable beloved member of their family.
Normally one can not come out of the 'Asubham' state within 12 days, which is a very very small period, when comparing with the days spent with their parents or any other elder members.

The Subha sweekaram procedure is only in "Sri Vaishnava Sampradayam".
Because, as per this sampradayam, one who did prapaththi once through an Acharyan, then at the end of this body (deha avasane) he / she (there is no gender for an Atma - all Jeevatmas are female only according to Vaishnavism) will definitely taken to moksham / parama padham immediately after his / her death.

One should realise the value and greatness of 'Parama padham' which can not be compared with any known places like "Swarga lokam", "Sathya Lokam" etc.
Also the 'Parama padham' is the origin and destination of Jeevatmas (including all castes, religion sect etc.).

So, one will really get more and more happy to reattain his / her origin or destination.

Now the departed 'Sri Vaishnavan' (or Prapannan) is going to 'Parama padham' to his origin and which is a greatest place of all.

So, the family members should really get happy instead of having sorrow.
This facts will be discoursed by a vidwan on the end of 'Satrumurai' on 13th day.
Being known this facts, subhasweekaram will be performed after that with much happy by celebrating that day.

Regarding "Dhadhiyaaradhanam":
"Dhadhiyaaradhanam" is nothing but a 'Vaishnava get together'.
Get togathers are performed at the eve of a hard task or a hard achievement etc.
Here also, performing 16 Masika, oona chradham and Apthikam without any default / fault is not an easy task.
The karthas should keep a hard vradhams like,
1. to keep beards (dheekshai)
2. do not have 'dhaampathyam'
3. do not celebrate any festivals
4. do not take outside / others house food
5. Should give preference to masyams than any other subham for even
close relations like, son / daughter marriage, seemantham etc.
6. Should not perform punya kaaryams, pilgrimages, tours etc.
7. Should not take thaambulam, Vasanai and other logiri vastus.
(Given only few important points).
I hope, you will now accept, that Keeping all these vradhams strictly and completing the Apthikam is very hard.

So, the 'Dhadhiyaaradhnam' is performed by inviting all Vaishnavite friends and relatives, informing that now he is free from all restriction and he is eligible to attend any function, and eligible to take any 'Virundhu' etc.

Also getting 'Asirvadham' from elders at the end of a 'Dhadhiyaaradham' is most important one.
So, this Asirvadham should be done only after the 'Dhadhiyaaradhana' bhojanam.

A great and long 'Swasthi' will be delivered by the 'Bruhaspathy' by showing all his talents in that.

In olden days, there will be a big mass to here the 'Swasthi'.
Now atleast 1% of the gathering is not at all minding about it.

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So, 'Dhadhiyaaradhaam day is purely a rest day without any bindings, the maximum ritual on that day is 'Asirvadham and Swasthi'.

There is not a single percent connection with 'Subha sweekaram' and 'Dhadhiyaaradhanam'.