Classical Athenians commonly ate just one meal a day. This meal was usually taken in the evening, and it consisted of two parts: sitos (staple) and opson (everything else). You were expected to have one with the other, and to take your opson in moderation. The word for "glutton" was opsophagos - "opson-eater".

In terms of sitos, the Ancient Mediterranean diet was very different from what it is now. Rice and potatoes were unknown; pasta had not yet been invented. The only staples available to the Classical Greeks were barley and wheat. Barley - the cheaper crop - was mixed into porridge or wet barley cakes rather than bread. This was the staple of poor people and Spartans. If you were a well-to-do Athenian, your staple on a special day would be wheat bread.

The opson would be where you'd get your flavour and your nutrients. If you were having guests over, it was common for every guest to bring one kind of opson, like potluck; they would all be pooled and your guests would be able to eat whatever they liked with their sitos. No doubt the most common accompaniment to bread would be lentils and chickpeas, possibly ground into some form of proto-houmous using olive oil. Onions and legumes were both used for food, although the former was stereotypically associated with warriors on campaign (Aristophanes regularly jokes about men marching out from the city smelling of onions). Sauces could be made of any ingredient using oil, vinegar, and possibly fish. Fried fish was also eaten on its own as a form of opson. The most common fruit - indeed, one of the four pillars of Ancient Greek agriculture, along with grain, olive, and vines - was the fig, which could of course be eaten fresh or dried (dates). Since you're looking to celebrate a special occasion, you might serve your guests some cheese; this was generally a luxury product, but one comedic reference to a stand in the market square exclusively selling soft cheeses suggests that there was plenty of choice.

What you would almost certainly not be eating is meat. In the diet of most Athenians, meat did not feature at all unless animals were being sacrificed in a public religious festival. Now, Athens was famous for staging lots of these festivals all year round, but meat would still have formed only a very small part of an Athenian's diet, and she would have little control over when she might get some. Probably the only meat product available in the market would be sausages, and it's not known how dear they would be.

To wash all this down, you would have wine. You would not drink it straight; that was the practice of northern barbarians, which (to the Greek mind) explained their wild and unrestrained behaviour. Civilised men drank their wine mixed with water. Slaves would pour wine, water and honey into a large mixing jar called a krater, and then pour out the mixture into large flat bowls called the kylix. While some of the places in Greece that were famous for their wine in Antiquity are still producing wine today (like the islands of Samos and Chios), I'm told that various crop diseases and centuries of refinement mean that today's grapes are completely different from the ones used in Classical times. In addition, we obviously can't tell how strong the mixed wine would still have been, though it would probably have been closer to modern beer than modern wine in terms of alcohol content.

While dairy was too expensive and grain likely too course for the Classical Greeks to have cake, there were various sweets that you might have for desert. The most common ingredients of sweet snacks were honey and sesame, both of which could be used in small cakes.

For more details on Greek appetites in all forms, I'd recommend James Davidson's Courtesans and Fishcakes.