Will I be able to board on Hikari Shinkansen at Oct, 26 daytime by my 1 week JR Pass if I activate it on Oct, 20?
I'm almost sure that the answer is 'yes', but asking just to confirm - as I'm planning an expensive trip on Oct, 26.
If you have a 7 day Japan rail pass, you are guaranteed to travel for 7 whole days. Then if you activate JRPass on Oct 20th, you can travel from Oct 20th 00:00 to Oct 26th 23:59.
When you activate your pass, they will put a large sticker on it mentioning the first and last days, and yes it is inclusive. The image below shows an activated 7-day JRP valid from 19 to 25 january 2017. That means the owner of this JRP could have used it with no limits including on the 19.01 and 25.01 dates.
"if I activate it on Oct, 20" makes no sense. If you mean "if I exchange it on Oct. 20", the answer is that it depends on when the validity period starts. If you mean "if the validity period starts on Oct. 20", then yes.