These are just fun little factoids about the numbers in this campaign and also japanese slang, especially online slang. So 3/9 is Miku day and one of the reasons I wanted to make this launch in March. This is not Miku's birthday (the day her software was launched) but another more celebratory day based around puns! 3 in japanese can be read a san or mi and 9 can be read as ku or kyuu. So you've got Mi Ku or San Kyuu. The first pun is obvious but sankyuu is a little tougher. It's a fun way to say thankyou! So you're both saying thanks or Miku with this date! That's why most of the pledges end with 39 because I wanted to keep the convention going. 

Now Teto's number doesn't fit the pattern and we have another number pun to thank for that. 8 can be read as hachi or pachi in japanese and lots of 8s together is often sounded out together as pachipachipachipachi. Like 8888888. Often at the end of shows the livestream comments will be spammed by lots of 8s which bring to mind the sound of applause and is used as a text version of such. As Teto is our last big pin I thought that would be fun! (Last for now, I do have some surprises if we do well enough.) 
8s are also lucky but that's neither here nor there.