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You will need at least 65 metal bottle caps, acrylic paint, fine paintbrushes, adhesive craft magnets, and paint sealant.  Be sure to have thick cardboard and a hot glue gun on hand. They may also be required. You may also use spray paint as a first "primer" coat instead of slowly painting each by hand. If the magnets are too thin, they won't be able to make contact with surfaces and stick properly. Glue squares of cardboard to the inside of the bottle caps if your magnets are too thin. Lay down a solid base coat and let it dry before adding details.  Paint at least 31 caps one color for the dates of the month. Paint at least 7 caps a second color, one for each day of the week. Paint at least 12 caps a third color for the names of each month. For special occasions, you can paint the remaining caps additional colors. Or, you can reuse one of the three basic colors. Use a fine brush to paint on your labels. Number each of your date caps 1-31. Write abbreviations on your month and day of the week caps. Paint symbols for special events (like balloons for birthdays or a heart for Valentine's Day). Without the sealant, acrylic can chip away. Wait for the paint to completely dry before sealing and then wait for the sealant to dry before moving on. A spray on sealant will take less time, but a brush on sealant will work fine, too. Cut the magnets into appropriate-sized squares. Remove the tape to reveal the glue side. Press the sticky side of a magnet either into the inside of each bottle cap or on top of the cardboard. The best thing about this method is how easy it is to change how the tiles are laid out. You can spread your calendar over a whole freezer or carefully line it up inside a locker. It's also incredibly easy to transport. Just put the pieces in a small box or bag, and you can take them anywhere.

Summary:
Gather your materials. Make sure your magnets are thicker than the inside of the bottle caps. Separate your caps into at least three different groups and paint them different colors. Add details to your caps. Apply the paint sealant. Attach sticky magnets to the bottle caps. Arrange your calendar however you want.