One important part of creating religions is creating the associated holidays. The biggest problem that appears with this is figuring out how to do that without just creating pseudo-Christmas or pseudo-Samhain.
The holiday should be directly related to the religion. This means that whatever holiday or holidays are being celebrated, they should tie directly into the lore. There are stories behind virtually every religious holidays that exist, whether they have changed since the holiday’s creation or not. Christmas is to celebrate the birth of Jesus; Samhain is to mark the end of the harvest as part of the Wheel of the Year; Duwali is to celebrate the triumph of light over darkness.
When you create your own religious holiday, make sure that it fits within the lore of the religion. If there is some sort of messiah figure, the date of his or her birth and/or death would probably be celebrated, depending on the lore. If there is a finite pantheon of worshipped gods and goddesses, they might each have a day, probably associated with whatever they are associated with. The harvest god(ess) would probably be celebrated around the start and/or end of the harvest, while the war one would probably be celebrated on an anniversary of a major military victory.
The holiday might be directly related to the time of year. As previously mentioned, a number of holidays celebrate specific times of year or changes in seasons. Wicca, for example, celebrate the solstices and equinoxes. Harvest beginnings and endings might be marked.
This does not mean that you need four holidays with each one falling on or around an equinox or solstice. You can have one per year that celebrates the longest day of the year, the shortest day of the year, the day when the main god(ess) created the world, the beginning of the calendar, the end of the calendar, the middle of the calendar, or whatever else you want.
A celebration might make sense for the middle of the winter because it is a dreary, awful time of the year, especially if you live somewhere cold but don’t have any sort of major heating in your house, and so it’s a chance for everyone to lighten up and be cheerful. On the other hand, a celebration might make sense for the middle of the summer because there is plentiful food and you have more time for celebration because there is longer sunlight.
The holiday should be directly related to the civilization. If it is an island nation with a strong fishing background, a celebration relating to fishing makes a lot more sense than a celebration relating to military battles. If the civilization has a strong connection to their leader (religious, political, or otherwise), the holiday might center around that leader or that position.
The celebration should be related to the holiday. Just as the physical celebration should be related to the religion (celebrating nature if the religion is nature-based, etc.), the celebration should also be related to the holiday that is being celebrated. If it is a start-of-harvest celebration, it might involve ceremonial planting of seeds. If it is an end-of-harvest celebration, it might involve a feast. If it is a celebration of light in the darkest, there may be a lot of candles. If it is a celebration of the birth or death of a messiah, there might be a recreation of that event.
Holidays change over time. As cultures shift and join or split, the holidays will change with them. A lot of Eastern Orthodox Christians celebrate Christmas on a different day than Western Christians/Catholics. The birth of Jesus was associated with the solstice so the Christians could pull in pagans who celebrated Yuletide at that time of year.
Whether or not changes happen, and how they happen, will depend on a number of different things. The more widespread a religion, the more likely that things will change. On the other hand, if there is strong leadership, such as the Holy See or the previous Islamic Caliphate, religious holidays and practices are more likely to stay the same over time.
Holidays can be whatever you want them to be, but you should try to make them make as much sense as possible within the civilization and religion you have created. Think of the holidays you know about and celebrate—and then make sure you don’t copy them.