Firstly it's absurd not to go to as many interviews as possible, as everyone has said.
Go to every possible interview.
Secondly the answer to your literal question couldn't be simpler: quite simply,
in the first instance, just wait a couple of days before replying anyway - you've already delayed a few days in that way -
and then when you reply, very simply type "Yes, that's wonderful, thank you very much, I am delighted to accept you offer of an interview. What dates would be possible from the { insert date here about 5 to 6 days in the future } for your convenience?"
Note that you do not in anyway, at all, have to state anything, whatsoever, about "why" you have mentioned a certain date (5 or 6 days in the future). Just state that date. That's all there is to it.
(If, incredibly, someone replies with "can you come earlier?" or "why can't you come until the Nth?", which won't happen, simply ignore it and say "I'm available in Cityname from Dayname, what is most convenient for your company" and leave it at that.)
Note that even after you absolutely secure a job, even after you are onboarding!, you must go to every possible interview.
Thus, your question in bold,
"How do I turn down an interview offer, even though it's perfect for me, so I don't have too many interviews to deal with in the same period?"
The extremely simple answer is
Don't.
The simple answer is
Never, ever ----- ever ----- turn down an interview, for any reason.
No more than you would throw away a lottery ticket.
It might be a winner.
If you literally just don't know the most business-like phrasing in English, it's just "certainly, what dates would be best for you from the Nth"
(You do not, in any way, whatsoever, for any reason at all, state or mention in any way, whatsoever, why you've stated the date of the Nth. In business English to organize meetings (of any type) each side just states the dates available.)
I note Daxxor that you mention
"I'm not a guy who can easily speak his mind to other people asking new stuff like an interview"
Now that the school part of your life is over, nobody much cares any more about your academic (or even in a way technical) skills. You'll either be wealthy or not in 10 years due to, in a word, your "business skills" or perhaps "business sense". The primary ingredient in that is: communication skills. I'd encourage you, particularly, to go to as many interviews as possible, each one of which will vastly improve your skills and experience in that area.