There is no month in calendar that does not have a festival.
There is not a month in calendar when there is not a festival.
Do the two sentences mentioned above mean the same thing?
Why or why not?
Yes, they do have the same meaning but they do not have the same tone.
What I say with tone is that when we talk we don't just spit words through our mouths, we express emotions, we change the tune of our voices.
And it reflects also in language.
To explain this, I will give you an example:
I wish you stopped hitting me.
That is pretty nice isn't it? I am only mildly irritated with your hitting, and I wish you stopped it. Now, let's have a look:
I wish you would stop hitting me!
Wow, beside the exclamation mark, we have a much more strong and concise writing. This means I am irritated, as the previous sentence, but now I'm nervous. The construction wish + would gives us this clearly annoyed view.
For your sentences I believe this is the case. The first sentence is more like an information about the year and its festivals. But the second one is almost an outflow of frustration, like "Ow heck, I only see festivals in this freaking calendar! What's the problem with these people?"
They mean the same thing; both not and no indicate negation.
However, the use of no indicates stronger negation.