How did Sumerians celebrate holy days?
Sumerian civilization spans over 4,500 years, most of it pre-historic, our understanding of their religious practices are often sketchy at best.
The Sumerians first seem to appear around 6,500 BC, a consistent historical record only starts around 3,000 BC so we can can really only make educated guesses at their religious practices. Even then, trying to nail their practices down is to take aim at a moving target. Gods changed names, practices varied between cities and ritual was not always consistent.
It is only around 2,300 BC in the start of the Akkadian Empire when we see a bid to recover and record earlier records and myths -- including those of creation, a distant time of a paradise on Earth and a horrific flood,brought about by the sins of some king.
The Sumerians seem preoccupied by law, were a patriarchal society and had a changing pantheon of Gods with very human traits to them. You may want to look up Enki, Enlil, Ninlil, Inanna, Utu, Sin and many many others. (Wikipedia has reasonable entries on them). Religious beliefs and practices changed through the centuries, but most cities might have their own gods and their own temples -- which also had numerous other roles. Religion and rulership were closely linked.
Sacrificing food to a temple was always praiseworthy (originally human sacrifice seems to have acceptible), Temples also had storehouses, and seem to have kept reserves against bad years. Women could serve as priestesses... or concubines for the gods.
There would have been religious processions, festivals at key times related to agriculture and the passage of the year. It also seems there were sacred marriages that were sometimes re-enacted by a ruler and a priestess... sometimes openly. Fertility, the availability of fresh water, and the passage of the year were all of critical importance.