Watch and act accordingly. Be careful, but it's not necessarily the case that "December's rubbish, August is crap" etc.
I had one trade last week, which went well. The action overall seemed good enough, I just didn't get any signals in the right circumstances. As DT says, ES yesterday didn't seem unusual, but again nothing I wanted to trade.
"Sideways" can be subjective, depending on your timeframe etc. Choppy daily can disguise nice trending on 15 minute and so on. Personally, I like to avoid it on my chosen view, but it is still worth watching because sideways action can be the set up for a good breakout.
My view is that a good set up is a good set up, regardless, although I do pay attention - if it's 16.00 GMT on a Friday, I'm not looking to enter a new trade. That said, look at ES on Friday 15 minute at 17.45 - a classic, basic reversal signal, with another to tell you to take profit just before the close.