If I were doing this, I would change your if statement to a Case Select method. Or I would create a string array with all the months in it (probably use option base 1 to make things simpler too). Something like this small example:
Dim arrMonths As Variant
Dim Sh As Worksheet
arrMonths = Array("Jan", "Feb", "Mar", "Apr", "May", _
"Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec")
If IsDate(Format(txtSchedule.Text, "mm/dd/yyyy")) Then
'Do code
And btw, you don't need to select anything to do what you are doing. If you leave out the Select in your code and just use the reference, you'll save time by having faster code. As an example (which also changes a cell's color to yellow):
'Instead of ActiveCell.Select, use
Activecell.Interior.Colorindex = 6
Hope this helps out to get you moving. Also, try to be a bit more descriptive. Like "I'm using a user form where you enter a date and based on that date it will find a specific cell and color it yellow. There is one sheet for each month."