There are two common views concerning the length of the six days of creation as found in Genesis One. One view says that the days are the same basic 24-hour days that we have now. The second view is that these are day-ages that refer to much longer periods of time. The second opinion usually allows for evolutionary forces to be the driving factor in the creation of the plant and animal species. Proponents of this approach note that the plants and animals in the days of creation generally go from the less complex to the more complex and in this general trend matches the theory of evolution. I strongly believe that the days of creation are 24-hour days. However, I want to carefully look at the facts as found in scripture and science in order to support this position.
Specifically, you ask only about the first day. Genesis 1:5 notes the end of this day with these words: "And the evening and the morning were the first day." The same pattern is used for the other five days of creation. Genesis 1:31 states, "And the evening and the morning were the sixth day." Since the exact same formula is used for each of the days (not only this closing statement but in the other elements as well), they must be equal to one another. Therefore, what one is, the others are.
Those who propose the day-age theory point out that the Bible sometimes uses the word, day, as a reference to an age or a large period of time. This is true but it does not give the interpreter license to insert an age into any place that the word, day, is used. The context of the passage must demand it. It does not do so in Genesis One.
One reason the day-age theory does not work is found in the word pattern that closes each day (as mentioned above). Each point out that "the evening and the morning" were that day. This is in accordance with Jewish reckoning. According to the Hebrew concept of time, a day begins with the evening when the sun goes down. This has been generally identified as 6PM. Roman time switches to the next day at midnight. Hebrew time does so when the day (as in daylight) ends. The evening comes first and then the morning. This kind of description only fits a 24-hour-day. It would make no sense if the day were thousands or millions of years long.
The proponents of theistic evolution (the idea that God gave the original bump to creation but that evolution explains most of the rest) like to point out the similarities in order in the six-day creation with the teachings of evolution. The earth is formed before life appears on it (kind of a no- brainer). Plant life comes before animal life. Sea creatures are created before land creatures. Man is last. What they avoid are the many dissimilarities and the outright contradictions to evolutionary order.
For instance, all vegetation is created (third day) before the sun, moon, and stars (fourth day). How did chlorophyll work for an entire age before the sun existed? Vegetation is also created (third day) before insects and birds (fifth day), but much vegetation depends on these creatures for pollination. How did the plants survive for two long ages without proper pollination? In another example, all life in the sea (fifth day) comes before the most basic life on land (sixth day). Specifically mentioned as created on the fifth day are the mammals that are whales (Genesis 1:21). These details and others do not fit the evolutionary scheme at all.
In the end, it comes down to whether or not someone believes the Bible to be literally true. If the transient opinions of science are someone's true authority but they are religious and want to make a gesture towards the Bible, then they may latch on to theistic evolution as a compromise. In this scheme, the days of Genesis One must be changed to day-ages and the text must be looked on as containing some truth in a mythical way but in no way being an exact account of truth. However, if one takes God's word to be the final authority, the day's must be normal days. Perhaps they were a few seconds longer than they are today, but they were equivalent to our 24-hour days.