How far out do my preparations start? How much assistance do I need in the kitchen? How sophisticated are my methods and logistics? What's the latest and greatest in cooking technology that I'd use to impress such a powerful figure? I'm a little curious about the menu but mostly I'm wondering about the work.
As a follow up question, who would Charlemagne invite to a feast? Would there be religious figures there? Members of the army? Family?
To begin: Unfortunately, due to a lack of direct sources (of which I am aware, at least), I can only answer some aspects of your question. If you were searching for something like a menu or instructions for a cook at the court of Charlemagne, I don't know anything about that or about sources that might deal with it, and I recommend checking out books that others have mentioned or might mention later. Also, since English is not my primary language, and this is a specific topic that I am only experienced at in German, please don't mind if some of my vocabulary or spelling might sometimes be a bit off.
Now, what I do know about are two sources regarding the royal estates in the Franconian Empire around the year 800 (which are obviously written in Latin). But keep in mind that there are a few possible interpretations of a feast for Charlemagne:
The two sources I'd like to cite here are:
- The Capitulare de villis (written around 800), an instruction of Charles for his servicemen how to administrate the royal estates. You can find a version of this online at this (german) website: https://www.dmgh.de/mgh_capit_1/index.htm#page/83/mode/1up. Now if you don't know Latin, I recommend looking for a translation within the list on this website: http://www.geschichtsquellen.de/werk/5514. It lists English, French, Italian and German editions and translations, so you might find something there.
- The second text is a part of the Brevium exempla, a collection of shorter texts that include and describe the building stock as well as the amount of cattle and equipment and a rough estimate of the annual harvest of some royal estates (fisci), composed in the early 9th century. You can find it online (in Latin) on the same website: https://www.dmgh.de/mgh_capit_1/index.htm#page/250/mode/1up. I am unfortunately not aware of a translation of any kind, but you mind find something if you search for it.
Now, while obviously nothing beats reading the original texts, I will try to compose some of the information given by them in a few paragraphs, to answer at least some parts of your question.
The court of Charlemagne was not a static thing like gouvernments today. Around the king, there was always a good amount of family, friends, advisors, vassals, guards, servants, soldiers, etc. that followed him around in different constellations while he travelled the country. Charles needed to take care of many different things: A good amount of time he spent at war, with the Saxons, in northern Spain, in Italy, etc. While at peace, he could travel around his country to show presence or spend some time at a hunt, but he also needed to regularly hold assemblys for all the important people, be it lays or clergymen, mainly at some of the most prominent palatines like Aachen.
This system could mainly be sustained because the Carolingian kings directly owned a lot of land in the Franconian Empire, scattered around in different estates, mainly in northern France, current Belgium and western Germany, but also in other regions. Some of these were given out to counts as a fiefdom, so that those counts could hire enough men to ensure peace, enforce the law or protect the border, but a lot of estates were administrated by a villicus directly in service of the king, who had to deliver eventual surpluses of corn, cattle or money to the royal court.
The sources cited above give insight how these estates worked: The main farm (villa or fiscus) and some smaller farms around it given to serfs mainly cultivated grain (rye, barley, oats, wheat) on a large scale. A lot of this was used as seed for the next year, the rest to feed humans and cattle. The Capitulare de villis talks about how the villicus had to maintain carts, drought animals and personal to deliver an eventual surplus to the royal court at the bigger palatines or for a campaign at the border. The texts also name a lot of other plants cultivated though (especially chapter 70 in the Capitulare): Different types of fruit trees (Plums, apples, pears, chestnuts, nuts, cherries, etc.), herbs (rosemary, sage, mustard, dill, parsley, etc.) and vegetables (lettuce, cucumber, carrots, cabbage, onions, leek, garlic, etc.) were all well known, and Charles advised his villici to grow them on every estate (if climatically possible, that is). Additionally, a description of the fiscus of Annappes (near Lille in France) in the Brevium Exempla (chapter 25) names a lot of livestock that was held there, which included: Cows, cattle, calves, oxen, bulls, goats, sheep, pigs of various sizes, chickens, geese, peacocks, donkeys and horses, etc. Before this there were also named some kitchen tools, including pans and cauldrons (it's pretty difficult to get these right though, since a translation may deviate from the original meaning of the Latin terms). At last, the Capitulare (chapter 34) features short instructions on how to make some processed food like bacon, aspic, wine, blackberry wine, vinegar, mustard, cheese, butter, beer and flour (obviously later used for baking).
While this is not directly a menu or an instruction for the kitchen, these sources at least give an impression of what kind of stuff was available for the court and of the logistics: Small estates everywhere, that seldom hosted the royal court, but delivered their surplus in food or in form of money to bigger palatines, at which resources were gathered to supply the court more often.
I found the references in this artical to be a good starting point to research pre/early medieval food: Charlemagne's Table: The Carolingian Royal Court and Food Culture
I can’t get the actual document to open since I don’t have access to that site through my Uni, but it may also provide significant insight based on the title and abstract.