There are other methods too. Alien methods (heheee [slick] ). Methods from the Dark Side 8[ ... Explaining is longer than doing.
First decide how big your calendar has to be. I take in my example squares of 100 pixels. I need six rows and seven columns. Then open a doc with exactly that size, 700x600 and with a transparant background.
And have your layers palette ready. Double click on the background to make it a layer.
Press D to set your foreground colour to black (assuming you want it black squares. If differently, choose one of your colours), Ctrl+A to select all and then Edit>Stroke to apply a thin stroke to the doc.
When you press M to take the selection tool, officially called the Marquee Tool, you can see in the Option Bar that there is a box for the Style. Click on the little arrow pointing downwards and choose Fixed. Now set the size of your square, here 100 vertical and 100horizontal.
Now click somewhere near the top left corner of your workspace, outside the doc. A selected square will appear in the top-left corner of your doc. Press Alt+Delete to fill it with black (or your foreground colour). Keep it selected.
Copy this layer.
Go to Filter>Other>Offset and fill the double of your size in vertically (here 200). A second black square appears as by magic.
Copy this second layer and press Ctrl+F to reapply the offset filter. You now have five rows: three black ones, and two white ones in between. Correct? No: you also have a third white row below the last black square.
Now click the arrow top-right of the layers palette and choose Merge Visible. Your three squares are on one layer. Copy this layer. Go to the offset filter dialog and fill in the size of your square for horizontal and vertical. (here 100 and 100) You now have a second row of your checkerboard.
Once again you merge these two visible layers and now you apply the offset filter 200 horizontally. Copy this new layer and press Ctrl+F once again, and repeat this a fourth time. The last squares fall outside the document.
To get rid of all the layer info that falls outside the doc, press Ctrl+A to select all, Inverse Selection and Delete.
Create one more layer, fill it with the colour of your fancy and place it at the bottom of the lot. Then Flatten Image.
Another method would be to make a pattern, but that is something someone else can explain. Patterns may be quicker, but this method I described is quick, and you do practise a lot of uncommon tricks.
Have fun!