Here is a picture of one of me seedling trays. As you can see I don't have any labels but lots of seedlings. The arrow at the top is my way of knowing what side is what.
You may have to expand this picture to understand it but this is the layout for the seedling tray. I have it named so I know which tray it is, in case I have more than one tray that have the same cells. I also have the date in which the seeds were sown. Each square represents one cell and has a number assigned to it. Below the diagram I have each type of seed listed by their number. You can see that the first row is all 1's and there for using the "legend" it is marigolds. Many of the squares also contain a date now. Some more than one. Each date is the day a seedling appeared. Some have a date and a x2 next to it. Meaning that day 2 seedlings appeared in that square. I know it seems silly to explain that but it helps me keep track of how long it takes each seedling to emerge. Some were consistent and all started showing with in days of each other and some have yet to even emerge.
Using this chart and date method allows me to keep track of not only how long it took for each seedling to emerge but as they don't all grow at the same rate allows me to use the information to plan the garden better. I can also use the information and see how viable the seeds are. If some of the seeds were harvested from my garden I know which are good or not. If they were purchased and are a few years old then I can see if they are still good.
It's also interesting to see that though 5 cells were planted with marigolds 1 cell doesn't have seedlings yet, but they call came from the same package.
When the seedlings get transplanted I simply cross out the cell they were in. I usually like to keep the sheets so that I can use them in following years as a guide. These seedlings worked well in this type of container or these didn't. Things like that.
It's important to have your own way of logging events. You may not care when the seedlings emerge just when they harvest. Or when the first true leaves appear. But as long as you keep some sort of log you will have something to help you in following years.