The tiruvaaraadhanam which is performed daily is called ijyaa. On special occasions like Sri Rama navami, Sri Krishna jayanti etc we need to perform special tiruvaaraadhanam in addition to ijyaa. This special aaraadhanam is performed at the avataara time. There is a small change in the one sloka which we recite at the start of mantrasana.
Daily, during ijyaa at the start of mantraasana we recite:
ijyaa kaalstR^itiiyo.ayaM ahnoM.ashaH samupaagataH.
saMbhR^itaashchaiva saMbhaaraaH kalpitaanyaasanaani cha..
recite:
loka naathasya raamasya navamii samupaagataa.
There is no change in the second line of the sloka.
On Sri Rama navami day perform special aaraadhanam during noon time.
recite
loka naathasya kR^iShNasya jayantii samupaagataa.
There is no change in the second line of the sloka.
On Sri Krishna jayanti the special aaraadhanam should be performed
at midnight.
loka naathasya nR^ihare jayantii samupaagataa
There is no change in the second line of the sloka.
The special aaraadhanam on sri nrusimha jayanti should be performed
in the evening/sunset time.
The recitation will be like this
chaapamaasaM gate bhaanau uShaH kaalo.ayamaagataH
saMbhR^itaashchaiva saMbhaaraaH kalpitaanyaasanaani cha..
Rest of the procedure and mantras are same.