Now is not the time to worry about your abs
Big picture, Crossfit is not a figure competition and these 5 weeks are NOT going to be the time we want to focus on trying to lose a few lbs. This might be the most stressful time of the year which is already working against your body composition. Lets not add to it by putting ourselves into a massive calorie deficit.
Fuel your performance
There are several key aspects to think about when it comes to your diet, but it all comes back to making sure you're recovering as much as possible from each training session, especially before you do an Open workout
High level: Is your training volume changing?
Many athletes drop their training volume dramatically the week before and throughout the Open to allow for optimal performance two times a week. Instead of doing 4 hours, they might reduce down to 2-3 and remove some intensity, with the obvious exception of 18.X each week.
We definitely do NOT want to make major reductions in carbs, but if your training volume has been cut in half, listen to your body. If you're having trouble finishing all of your macros, talk with your coach about making a small reduction. Emphasis on SMALL.
Pre Workout nutrition is a 48 hour process
The reality of this situation is that it takes 24+ hours to actually replenish glycogen stores, which means that if you want to be "topped off" for 18.1, you need to make sure you're eating enough TODAY and TOMORROW (24-48 hours prior). Its probably a good idea to add in an extra 50-100 grams of carbs to your typical macros the day before you take a run at the Open workout. Don't go crazy on fiber here either... we want straight forward carbs that the body can actually use without literally digesting it for 2 days. Veggies are great, but not so many you look like a rabbit.
Also, stay relatively hydrated. This does not mean you should drink so much water that you think about "recycling" your piss. Just make sure you're not thirsty before you go to bed. You can't fix dehydration in one hour, so get your shit together today.
Don't lose your mind and try something completely different from normal. If you normally train fasted at 6 AM, Friday is NOT the time to decide to eat a full meal 2 hours prior and hit the workout at 6 PM. Not unless you want to feel like a complete dumpster fire, at least.
For higher level athletes, follow the usual competition routine. Eat a solid protein and carb based meal with easily digestible foods 2.5 hours from "3, 2, 1, GO." This will ensure a fairly empty stomach and prevent hunger from being an issue. 30 minutes prior, if you typically take BCAAs, take your BCAAs. If you typically take Beta-Alanine, take Beta-Alanine.
If you've NEVER done either of those things before, NOW IS NOT THE F-ING TIME TO TRY IT. Same thing with coffee. If you don't typically drink caffeine, do not start now unless you want to end up in a pile of your own urine, vibrating on the ground with the feeling of your heart wanting to leave your chest.
I made this mistake at Regionals in 2010 (yes I'm pretty old). I happily chugged a coffee 60 minutes before a workout of 3 rounds of 20 box jumps, 20 C2B and 20 wall balls. Warming up, I did 3 box jumps and felt my heart rate go to 100%. "Mistake have been made..." was my only thought but there was nothing I could do about it. Don't be an idiot like 2010 Mike... be better.
***SIDE NOTE: Oftentimes higher level athletes will use a session in the morning to work out the kinks for an Open workout later in the afternoon. Have a SOLID amount of purely liquid carbs after the first session. Think 50-100 grams worth depending upon the size of the athlete, and the duration/intensity of the first session.***
The open is a highly stressful process. Oftentimes, stress can shut down aspects of the nervous system that control digestion making it really hard to eat. With that in mind, eating really simple foods can make a huge difference in your ability to recover. Veggie are great, but not when you're struggling to get down a meal post workout. Think about using a protein and carb powdered shake POST WORKOUT to make life easier on your gut. For most people 20-25 grams of protein and 25-50 grams of carbs will be a good idea. Side benefit: that blood sugar spike that this post workout drink will give you will also lower your stress hormone (i.e cortisol) and allow your body to start recovering faster... because ya know you're gonna lose your mind and try the damn workout a 2nd (or 3rd) time.