Do you mean that you have trouble dealing with the data that is returned by loadStrings()?
Or... is it difficult to read from a text file whose name is constantly changing?
In the case of the latter, then...

Try creating another text file with a name that stays the same that contains the names of the text files that change from a day-to-day basis. You can update the text file every day to accommodate for the changes in the other text files' names.

The problem is to get the name of the file?
Here is a simple sketch illustrating how to do it.

// Change path to one on your system
final String PATH = "H:/Temp/Other"; // final = won't change, constant
File dir = new File(PATH);

// Get the list of files
File[] files = dir.listFiles();
println(files);
String toOpen = null;

// Look at the file names
for (File file : files)
{
  // Check extension
  if (file.getName().endsWith(".txt"))
  {
    // Get path. Note: I could also make a file filter to give to listFiles
    toOpen = file.getAbsolutePath();
    break;
  }
}
String[] lines = new String[0]; // No need to check for null with this trick
if (toOpen != null)
{
  lines = loadStrings(toOpen);
}
println(lines);


// Alternative

dir = new File("H:/Temp"); // More files there...
files = dir.listFiles(new FileFilter()
{
  public boolean accept(File file)
  {
    if (file.isDirectory())
      return false; // Only files. A directory can have a dot in its name...
    String name = file.getName();
    return name.endsWith(".txt") || name.endsWith(".xml");
  }
});
println(files);

exit();

Advantage of the second form: less memory is allocated if there are lot of files and filtering is aggressive.

I assume the daily data comes in one file per day.

Find out the exact names of those files.

Is it

2012_05_12d.txt

that would be for today

or is it a number
2324231.txt
?

Then go and reproduce the name with a rule (the same rule the names are made by).

e.g. for file name based on the date:

When you want to have all text files from the 1st jan 2012 to today (May 12th 2012) you would create a function to increase a date and make a file name from there and load them ony by one.

Here is a solution just for one day, today.

1. // loadStrings(var)                  //Because for every day ill have a different txt file.
2. void setup() {
3.   size (300, 300);
4.   // PFont metaBold;
5.   // metaBold = createFont("fonts/Meta-Bold.vlw.gz", 44);
6.   //setFont(metaBold);
7.   // noLoop();
8. }
9. void draw() {
10.   background(0);
11.   int d = day();    // Values from 1 - 31
12.   int m = month();  // Values from 1 - 12
13.   int y = year();   // 2003, 2004, 2005, etc.
14.   String strDay = String.valueOf(d);
15.   text( "Day " + strDay, 10, 28);
16.   String strMon = String.valueOf(m);
17.   text("month " + strMon, 10, 56);
18.   String strYear = String.valueOf(y);
19.   text("year " + strYear, 10, 84);
20.   strMon =  leadingZero(strMon);
21.   strDay =  leadingZero(strDay);
22.   //
23.   String myFilename= strYear+strMon+strDay+".txt";
24.   //
25.   text("File: "+myFilename, 10, 116);
26.   println (myFilename);
27.   // loadSomething(myFilename);
28. }
29. void loadSomething(String name1) {
30.   String lines[] = loadStrings(name1);
31.   println("there are "
32.     + lines.length
33.     + " lines");
34.   for (int i=0; i < lines.length; i++) {
35.     println(lines[i]);
36.   }
37. }
38. String leadingZero(String a) {
39.   if (a.length()<2)
40.     return "0"+a;
41.   else
42.     return a;
43. }

yeah, I had the Code from the help on month().
It's strMon = nf(m, 2);

1. // loadStrings(var)         //Because for every day i'll have a different txt file.
2. void setup() {
3.   size (300, 300);
4.   // PFont metaBold;
5.   // metaBold = createFont("fonts/Meta-Bold.vlw.gz", 44);
6.   //setFont(metaBold);
7.   // noLoop();
8. }
9. void draw() {
10.   background(0);
11.   int d = day();    // Values from 1 - 31
12.   int m = month();  // Values from 1 - 12
13.   int y = year();   // 2003, 2004, 2005, etc.
14.   String strDay = nf(d, 2);
15.   text( "Day " + strDay, 10, 28);
16.   String strMon = nf(m, 2);
17.   text("month " + strMon, 10, 56);
18.   String strYear = str(y);
19.   text("year " + strYear, 10, 84);
20.   //
21.   String myFilename= strYear+strMon+strDay+".txt";
22.   //
23.   text("File: "+myFilename, 10, 116);
24.   // loadSomething(myFilename);
25. }
26. void loadSomething(String name1) {
27.   String lines[] = loadStrings(name1);
28.   println("there are "
29.     + lines.length
30.     + " lines");
31.   for (int i=0; i < lines.length; i++) {
32.     println(lines[i]);
33.   }
34. }