I travel frequently for work - occasionally it's optional, though mostly it's not - and have some thoughts, though I have to admit I've never spent 6 nights away from the boys. My schedule is more like being away 2 - 3 nights a week, every week.
What I discovered early on was that it was best not to give either of the boys too much lead time in terms of telling them about my departure - the younger they are the more confusing it is for them, given their elastic view of time, and the older they are, the more likely they are to use that extra time to whine/complain about Mom leaving. In general, I tell them the night before I leave - if it's an early morning departure (like, a 6am flight for which I need to leave the house by 4am), I tell them at supper time. If they're going to see me in the morning, I tell them at bedtime, and then remind them in the morning. It may sound like not much notice, but telling them earlier created a lot of angst and confusion and distress, and over 7 years of this I've discovered short notice is much easier on everyone.
The difference is if I'm going to be away for part of a weekend (this is rare, but it happens occasionally) - then I give the older one a heads-up much earlier. DS1 is almost 8 now, and I make it a real point to get in an activity just for the two of us over each weekend - usually Sunday afternoons. If this is going to be impacted, or if my absence is going to be extended, I will tell him a few days in advance and brainstorm with him about what fun thing we could do together in the meantime, so he doesn't feel cheated on his "Mom time". DS2 is only 2, so I still just tell him the night before.
In terms of presenting the information, I always just say "Mommy has to go away for work." I never tell them when it's optional and when it's required; I tried this with DS1, but honestly, the younger they are, the fewer details they need. Plus, if they hear you don't HAVE To do this but just think it's a good idea - well, that's hard for them to wrap their heads around. Better to just be clear that you're going, and that you're coming back.
To be clear that I'll be gone overnight and/or leaving at the crack of dawn, I tell them I am going on a plane. I am careful to tell them exactly how long I'll be gone - how many nights - and what I will do first thing when I get back. ("I'll get home after you are asleep but I promise to come in and give you a kiss and a cuddle as soon as I get here!" or "I will miss dinner but I'll be here in time to give you your bath and read books before bed!") For the little one, I also reassure him by telling him his dad and brother will be with him, that he will still see his regular DCP during the day, and his favorite babysitter will come by the house, too. In other words, his routine won't be disrupted and he'll still be with people he loves. We also have a tradition that they call me at bedtime when they are cuddling with Dad - and every time I tell them I'm going away, I extract a promise from them that they'll call.
We also co-sleep, both boys are very tight with DH, and DH has a flexible work schedule that allows him to spend more time with the boys when I am gone. We also have a regular babysitter whom both boys love, who comes a couple evenings a week to give DH a hand. So I think all these things help.
Good luck! Let us know how it goes.