How to Make Garum and Why You May Not Want To

Garum or liquamen is a fish sauce ubiquitous in Greek and Roman cooking, and common to varying extents in other Mediterranean cuisine, where it is often called muria as well. It is made by cutting up various fish and fermenting them with salt over a long period of time, as described by Gargilius Martialis in De Medicina et de Virtue Herbarum:

Use fatty fish, for example sardines, and a well-sealed container with a 26-35 quart capacity. Add dried aromatic herbs possessing a strong flavor, such as dill, coriander, fennel, celery, mint, oregano, and others, making a layer on the bottom of the container; then put down a layer of fish (if small leave them whole, if large use pieces); and over this add a layer of salt two fingers high. Repeat these three layers until the container is filled. Let it rest for seven days in the sun. Then mix the sauce daily for twenty days. After that tie it becomes a liquid.

As may be observed, the production of garum is both lengthy and noisome. Contemporary writers often noted the pungent smell of the fermenting fish, and it could not considered polite to produce it anywhere with nearby neighbors. Nonetheless, it was a crucial part of ancient cookery, with many different recipes and grades of sauce recognized. Martial 13 notes that mackerel is a preferred fish, and anchovies are highly regarded, whereas more plainly flavored fish seem to be less prized. Although the prospect can be offputting to modern diners, its role in cuisine was similar to Worcestershire sauce, with small to moderate amounts being employed to add a savory quality to a wide variety of dishes, from meat to fruit.

Given this ubiquity, the cook of ancient foods must use something in the place of garum. Making it in the traditional style is a daunting prospect for most of us, and shortcut methods are unlikely to produce a very accurate reproduction. Worcestershire can substitute for garum, but it is not ideal due to the many other flavorings it contains. Southeast Asian fish sauces, however, tend to be made very much as garum was, and they can generally be found at any Asian grocery. I like to use the Vietnamese nước mắm, but other kinds such as nam pla or teuk trei can be used somewhat indiscriminately. The sauce you use should have a texture slightly thicker than soy sauce, with a flavor and aroma that is somewhat fishy but not overpoweringly so.

If you would prefer to make your own garum in the classic style, by all means do so. While I live in the city I will leave such endeavors to others.

Things about Feast Planning I've Learned the Hard Way

1. Do a test feast. You may not be able to do every dish as it will be done at the event (you can't spit roast a whole pig, for example), but you'll learn what works and what needs tweaking, and you'll have recipes you can confidently hand to your kitchen staff. Be sure to take notes when you're cooking the test feast, so you can make sure the final recipes accurately capture your process.
2. Use the skills your volunteers have. If you know someone is a veteran feast cook, you can set them at a dish and stop thinking about it. If someone is brand new but eager to help, you should give them very specific instructions on what to do - there are useful jobs for everyone.
3. Reduce the vagary of tasks. While you will have people of varying skill levels, you're more likely to get a large amount of unskilled labor than skilled labor. The more exact your recipes are, the less expertise a volunteer needs in redacting recipes.  
4. Put up large sheets of paper listing the order of the dishes, and what goes in each. This will let anyone in the kitchen easily see what dishes need to be served when, what dish their current project is destined for, and what potential allergens are in each dish. You could even use one such list as a checklist of what's been cooked/plated/served, making it easier to identify what needs attention right now.  
5. Map out how to cook your dishes with the tools you will have. If one thing needs to take over all your available ovens for three hours, you need to have worked out in advance when it can have those resources. To do this, you need a thorough understanding of what equipment you have at the site, and whether it has any quirks (ovens that don't work, sinks that aren't hooked up, etc.).
6. Plan to be flexible about your starting time. Forces beyond your control may well push the feast later, or in rare cases you may be asked to move the start earlier. This can wreak havoc with any feast, so the least you can do is think through some contingency plans in advance.  
7. Work out how long each dish can stand. If you're making hot fritters, they either need to go out right after they're made or you need warming ovens to keep them in. If you try to serve them in the first course this can make your timing very tricky, especially since you probably won't be able to control exactly when you start.  
8. Do things that can reasonably be done in advance, in advance. If you need 20 pounds of chicken shredded, or 18 heads of garlic peeled, think about doing them ahead of time. Your volunteers will be happy you did, and it will remove work that otherwise could become a bottleneck on the day itself. You generally don't need to chop vegetables in advance, or pre-cook meat, or anything else that's best when it's fresh - just anything that's tedious, and can sit for a day or two. 
9. Be realistic about what people will eat. (Or, Nobody Eats the Barley.) This is notoriously hard to get right, but there are some tendencies you can work with. People will eat lots of bread and meat, a fair amount of stew and pie, not that much fish or vegetables. If something will seem weird to the average palate of your diners, make a lot less of it. If people love it at the test feast, AND the test diners are a good representation of the real diners, make plenty of it.  
10. Remember that people get full. Everyone is hungry when the feast starts, so they eat a lot of the first course, and less of each subsequent course. You can use this in various ways: you can put cheaper foods earlier (to a degree. Don't put the best food out when no one can eat any more); you can entice people to eat weird foods by serving them first, though if it doesn't work you'd better be quick with the second course; you can keep all the courses small so people stay hungry longer, as long as you are very sure of your portioning and the timing of your courses. It can be risky to rely too much on any of these approaches, but it's something you need to be aware of. People simply will not eat as much meat at the end as at the beginning, all else being equal.