Someone asked this question today, and the answer is a resounding yes.
Being a lawyer means that you never, ever sleep well. You wake up in the middle of the night wondering if you really did put the proper mailing address on that letter you sent to a credit who kept contacting your client after the bankruptcy was filed. I also receive email court notices every morning at 12:06 a.m., and I wake up to check them every single morning.
This means that I usually check my other email as well, and yes, if a client has sent me an email on a Saturday, I try to do the decent thing and respond.
I had a potential client email me Saturday morning at 12:30 a.m., and I sent a reply at 2:30 a.m. I haven’t heard back from her yet, and hopefully my early morning responsiveness didn’t scare her off.
So in short, yes, at least one lawyer responds to client email at night or over the weekend.