You can sell all or part of your Core ETF portfolio at any time. However, it's essential to remember that investing in ETFs is generally considered a long-term strategy. While it's natural to feel anxious when markets are down, making decisions based on short-term market fluctuations can be detrimental to your long-term investment goals. Historical trends show that markets tend to go up in the long term. Hence, it's advisable to stay informed, remain patient, and avoid making decisions solely based on emotions. Successful investing is often about time in the market, not timing the market.