You could take it the day before. No worries. These are bacteria normally present in your intestine anyhow.
Taking it the same day would be a waste. You will flush it anyhow.
Drink loads of water today so as to prepare for the Liver Flush (it can be hard on your kidneys).
Don't worry too much about the symptoms. The Magnesium in the Epsom Salt is beneficial for the intestine furthermore, the flush involved in a Liver Flush just drains it all out (bacterias good and bad). Do consider doing an enema the day following the liver flush. And make sure you take your acidophilus the day after the flush.