I am getting too old to get up in the early, early AM hour to get a brisket flat smoked for a lunch gathering.   Has anyone done this the day before and reheated it someway without ruining it?

The best way I have found to reheat most anything I cooked in advance or extra from the freezer, is sous vide. The only drawback in reheating is you lose the bark, if that is important. You could drop it in some hot to boiling water for a while also.

I've cooked brisket the day before and kept it in the fridge after cooling down enough overnight and then reheated it in the oven after transporting it to the eating place the next day.  I did 250 degrees for about an hour after preheating the oven.  I've also done it in the pellet grill the same way here.  Cook it the day before and then just reheat in the grill if you like the smoke smell for the guests.  This was for 10 to 12 pounders in a foil pan with some liquid so it might be less time for a flat to reheat.  They were put in the pan and covered with foil at around 165 degrees with some beef broth until probe tender and then never opened until reheated and ready to slice.  I need to learn how to do a flat like that too.

A caterer I know puts wrapped brisket in a warmer at 145*F to hold it for as long as a day.  I saw the warmer.  It's stainless steel and about the size of a small refrigerator.

All the time.

Do you slice it before reheating or after?

I slice it after cooking, before reheating and reheat it in vac bag and boiling water if possible.

Do you slice it before reheating or after?

I've sliced it after but I don't think it really matters.  If you have guests over, the visual of it being sliced might be more pleasing to them but that is just a guess.

On another note, about how long and what temp do you cook a flat or a point?

For me, length varies depending on pit temp.  Do you mean cooking temperature or IT of meat?

On another note, about how long and what temp do you cook a flat or a point?

Cooking temp.  The two I've tried were tough because I think they cooked too fast.  They were both in my old upright electric smoker before my pellet grill days.

I have cooked great briskets at 225° and I have cooked great brisket at 325°.  The old adage its done when its done is true, but for me, a finished brisket will usually be tender when it is in excess of 205° IT!  Does not seem to matter to me how long it takes to get there!

Ditto.

I probably pulled them too soon.  Now I get them up to 205 thanks to the vast knowledge on this site and they come out good.  I didn't have a meat thermometer back then and went by looks.

Do you slice it before reheating or after?

I've sliced it after but I don't think it really matters.  If you have guests over, the visual of it being sliced might be more pleasing to them but that is just a guess.

On another note, about how long and what temp do you cook a flat or a point?

I like to smoke them low and slow .  I start out at 180 and then bump it to 225.  I use a base of 1 1/2 hour per pound, but they are all different.  I also like to let it rest a minimum of 1 hour.  It seems like I always have to rush it to get it done on time if I smoke on the holiday itself.  When I do it just for my wife and myself I let it take how ever long it takes.  But when you have a group of people waiting at a set time I sometimes have to rush it.  Thus the reason for asking the main question in this thread.

 Just thinking?   The next time you are cooking a brisket or pork shoulder for yourself try a higher temperature: 275º to 325º.  You may very well be pleasantly surprised

how well it  turns out. If not you will have tried & can blame me.

 For years in the low & slow camp. Nice enjoyable long cooks a slow moving column of smoke emerging from the pit. Very relaxing to me. Then I was confronted with a

similar situation as you describe here.

 I took a chance, on the next cook raising the cook temperature as I had miscalculated the cook time that I had planned on. The choices were either eat much later or

gamble using a higher cook temperature that could end up with an inferior result.. The result was = to low & slow. It was relaxing to have the cook done with time to spare.

 I prepped the day before, cooked & ate the next day. No all night or very early morning cooks.

 What drove this higher cook temperature cook method was meeting with an Atlanta Ga restaurateur serving very juicy & tender brisket.  I had to ask how he cooked it..

 The response was 325º on his komadoes.. He gambled on the temp switch & found the only difference was time. He could now get a good night's sleep. It was a game

changer.

 A word on DONE TEMPERATURES. I have been mislead several times by my thermometers, leave-in or probe. They tell the truth temperature wise, not done wise. My first

experience; I was reading a good 5º short of the target temp on the leave-in when for some reason I decided to check using the probe thermometer..Surprise; everywhere I

probed the feel was that of probing apple sauce.. It was done. Never looked @ the probe thermometer.. Probing tells the truth.