Could anybody tell me when this year summer season will be on the legendary banner

I assume between the end of this month and the next, assuming they have Sanaki and Ninian next.

If they follow the same trend they've been using for the last several months (they've only had 3 seasonal units in each of the last 4 months' Legendary Banners), we're likely to see the first Summer Unit appearing in this next banner, alongside Bridal Sanaki and Bridal Ninian. The other 7 swimsuit units, then, will probably be spread among Nov, Dec, and Jan's banners, with the Hoshidan Summer units returning in Jan and Feb.

It's possible, of course, that IS will do something weird with the spacing, which could spread the units out even further. But, for a quick estimate, the Oct-Jan banners is what I'd guess.

I'm still taken aback that they didn't run B!Ninian yet. It's anything goes at this point.

Depending on if they want to put more than 2 seasonals (Bridal Ninian and Bridal Sanaki very likely) on October's Leg. banner, we're going to be getting them starting October or November.