Short answer
Chandrashtama (Moon in the eighth) is the 2 to 2.5 days each month when the transiting Moon passes through the 8th sign from your Moon sign. For a Mesha Moon that is whenever the Moon is in Vrischika; for Simha, whenever it is in Meena. Tradition keeps these days for lighter plans and avoids big starts.
How ChiChakra reads this
The Moon moves through one sign in about 2.25 days, so it reaches your 8th sign once a month. We count from your birth Moon sign and give exact start and end times. Those rarely fall at midnight, so a Chandrashtama spell can begin mid-afternoon and end on the morning of the third day.
What to do on those days
Keep them for routine work, admin, rest and finishing things off.
If you have a free choice, don't schedule a launch, signing, house-warming or first family meeting then; moving it by a day or two is enough.
If something important can't move, such as an exam, interview, hospital appointment or flight, go ahead. Prepare well, leave extra time and keep conversations calm.
Some people feel more tired or sensitive on these days, so an earlier night does no harm.
Good to know
Everyone with your Moon sign shares the same Chandrashtama days. It is a gentle timing preference, not a warning of danger.