Welcome! Today I shall take you on a adventure, if i may. To explore the trills, frills and failures (and success) in my quest to build a dodecahedron calendar as a Christmas gift for my parents.
There is a bonus short story if you get to the end!

So i recently came across dodecahedral calendars. First over here
https://www.instructables.com/id/Dodecahedron-2015-...
After looking around the web for a good template to start with, found this site and used the blank template
http://craftmeister.mcuniverse.com/2014/2015-dodec...

I put my own design in the template, i will quickly go through it in the next step.

After making the first "prototype" purely out of A4 paper, I found it to be too fragile. I made the next with cardboard backing (will cover 3 ways to do this). Then i figured it is so strong we don't need to seal up the dodecahedral and it can still stand well, so we can also make use of all that space inside.

This ible is more for giving idea and tips as well as sharing my experience on this project. Its a rather straight forward project, but also much more difficult than I thought (for me at least)

Step 1: The Template

You can head over here to get a template and print it out if you like their design, or get the blank one and draw your own stuff on it.
http://craftmeister.mcuniverse.com/2014/2015-dodec...

If you like my design, feel free to use it.
I know its not that great but I am satisfied with it =)

Editing of the pictures were all done in Paint.Net, which i find to be an excellent tool. It is easy to learn, powerful enough for most task and free! (no they did not pay me to advertise)

What i did

I deleted all the white area inside the pentagons so that its transparent
I thicken the borders with purple lines.
next I made a new layer for the design, so if I mess up, i can delete this layer and there will be no need to do steps 1 and 2 again.
For my design i wanted it to be simple but not too plain.
1. For this i painted each pentagon with a colour (following the colour wheel)
2. Then i painted the each edge with a colour in between the pentagon and the adjacent pentagon.
3. repeat for all edges of all pentagons
Next is to transfer it to a doc file and split it to two pages, make sure to choose the right size of paper matching what you are going to print it on
Print and cut!
Crease all the edges.

You will be using these regardless of what reinforcing approach you choose to follow.
Initially I just glued the tabs and then the two halves without reinforcement. But it proves to be too flimsy and easily damaged.

In the following steps we will look at the advantages and disadvantages of each reinforcement method as well as how to construct it.

Step 2: Reinforcement Type 1: Individual Pentagon

Advantage:

Forgiving, you can trim each pentagon individually to fix perfectly
Easy
Decently strong

Disadvantage:

Weaker than the other 2 methods
Require more adhesive
a little tedious

For this method, here is what i did:

Glue the tabs to build the 2 halves out of the paper print outs
Cut 12 pentagons out of thin cardboard. I use cardboard from a calendar (how ironic)
Place each pentagon into the "bowl" like halves and trim to fit
Glue together with hotglue, i find hotglue works well because its easy to work with and cures to be quite tough
Once done for both halves, join them together (its harder than it sounds)

Extra tip on joining halves, if you are using hotglue, you can try making a "brush" to apply the glue on the inner surfaces.

Step 3: Reinforcement Type 2: Thin Cardboard Clone

Advantage:

Easy to do
Allows easy joining of halves (if use blank
Allows you to change the calendar

Disadvantage:

May require extra template "blanks"
May damage calendar if omit blanks

*so sorry for the lack of photos, completely forgot to document this part

For this method:

Print a blank or the calendar it self.
Paste it over a thin cardboard and cut it out along the line (if using the blank, use a weaker glue)
Glue the tabs and join the two halves
If you used the blank, remove the blank and use tape to strengthen the joints
If you used the blank, paste the actual calendar (double sided tape works best)

*Do not close up the last piece if you want to make it a storage box/desk organiser

Step 4: Reinforcement Type 3: Thick Cardboard Clone

Advantage:

Very strong (seriously, for cardboard at least)

Disadvantage:

slightly harder to adjust

For this part I will share with you my initial failure and how i fixed it.

Initially I preformed the steps exactly as in Reinforcement Type 2, however this is not a very good idea. The cardboard is quite thick, so gluing the tabs will cause the pentagons to be skewed and the whole dodecahedral will be very messy.

Instead, here is what you can do.

Cut out the cardboard as you would in Type 2, but without the tabs.
Instead of gluing the tab, hold the pentagons together and hotglue
Glue the two halves together.

*Do not close up the last piece if you want to make it a storage box/desk organiser

Step 5: So, What Now?

Here are some awesome ideas of what you can do with your dodecahedral calendar.

Dodecahedral Calendar Table Lamp/Torch
Dodecahedral Calendar storage box/desk organizer
Dodecahedral Date and Time
Many more!

Step 6: Dodecahedral Illuminating Date

Here is what i used

small tricycle with a basket from a gift shop (RM5=1.45USD)
Bicycle headlamps (RM 5 for 2)
Multicolour keychain light (RM 1.5=0.45USD)
Dodecahedral calendar (Priceless)

Basically, strap the headlamps on the tricycle and drop the keychain into the basket and rest the calendar on basket.

Step 7: The Bear's Treasure

Here we make a storage box/desk organiser.
Remember not to close up one pentagon. If you did, might want to get the knife out.

All we have to do is make a lining for the inside. To do this, cut out a half of the dodecahedral from your lining material of choice and stick it on the inside. You can also make 6 pentagons to line the rest of the inside but its usually not visible. You should however make a pentagon for the lid