Step 1: Recall Kashyapa Samhita on infant care.
Acharya Kashyapa lays down a schedule for introducing a newborn to environmental stimuli
Step 2: Recall the timing.
According to this schedule, showing the infant the Sun and Moon is recommended once the child completes the first month
Step 3: State the answer.
So the correct timing is the first month
\[ \boxed{First month} \]