Take one picture of each student, to have a child's picture for each weekday of the current month. If there are 20 weekdays in the current month, have 20 pictures taken. If there are less than 20 children, add a few group shots -- including the teacher -- to make up the extra weekdays. If more than 20 students are in the class -- although this is not likely in a preschool class -- then include the weekend squares. Print the pictures.
On a magnet board, use a dry erase marker to make the calendar squares for the month. Make the squares large enough to hold a wallet size photo. Write the name of the month at the top of the board. List the days of the week in different colors at the top of the appropriate calendar squares. Have the preschoolers count with you, as you number the days of the month with markers on the squares.
Give each child their picture, one at a time, and have them place it on a weekday square. Use a magnet to hold the photo on -- covering the number. Use the weekend squares if necessary or desired --with individual or group shots.
On the first weekday of the month, have the student whose picture is on that day's square remove their picture, revealing the day of the month for the class. Note: If any weekend days have passed with pictures on them, let the students whose pictures are on them, remove their pictures first. Post the removed pictures on a class bulletin board until next month, then use them again -- or take new pictures.