Increasing Cash Position on Monday

I've always followed the whole let it ride in an ETF mantra. But I manage my mom's retirement acct and she's about 1-2 years before retirement. I'm thinking of converting 1/2 of all S&P500 ETFs to cash on Monday.

Knowing that there's going to be a sell off this Monday, is it too late? Will my sell orders even get fulfilled? These are all long positions so I'm not worried about taking less green from the growth.