Photoshop

Load Files into a stack. Size and arrange the 12 layers.  Or  you could use a script like PasteImageRoll to tile your images onto a canvas. It is in my  Free Photoshop Photo Collage abd Mockup Toolkit  here is Help on that script Paste Image Roll script 

If you'd like to have the pictures automatically arranged without overlapping, you could use Contact Sheet II. You don't need to start with a new document. This feature will create the document for you.

1. Go to File > Automate > Contact Sheet II... 
2. In the Contact Sheet II dialog that opens, set the Source Files. If they're not open, click Browse... to navigate to your images.
3. In the Document section, set the size that you want the document that contains the image to be and probably don't Flatten the layers in case you want to move the content around.
4. In the Thumbnails section, set how you want the pictures placed. If you want it to look like 12 days with 7 on the top and 5 on the bottom, then you could set it to
   • Place: across first
   • Columns: 12
   • Rows: 2
   • (I would also recommend using Auto-Spacing)
   • Another option would be to equally distribute the images in rows and columns in which case change the rows and columns however you like.
5. Unless you want to include the filenames, I'd turn off the Font for Use Filename as Caption.
6. Click OK to have Photoshop build it for you.
7. Use the Selection Tool to move any of the images as desired.