Dead Heat Summer Race! Wiki

Summer Racing Festival: Ishtar Cup
Dead Heat Summer Race! ~The Ishtar Cup of Hopes and Dreams 2017~

While Master enjoys a well-deserved break with Mash,
Ishtar shows up with a big, too-happy smile.
She informs the pair that she found a new Singularity,
and warns that the world will be in danger if something isn't done.
After stopping Mash from requesting an official Order,
Ishtar mentions a certain ritual of her devising to eliminate the Singularity.
A summer festival to get everyone all fired up will be just the thing to deal with this Singularity...
That's right...the only option is to hold a cross country race!

A limited-time Summer event in Fate/Grand Order, released on the Japanese server on August 9, 2017 and on the NA server on July 29, 2019. The second half of the story is covered in Death Jail Summer Escape. Ishtar (Swimsuit Rider) is given as a reward to players for completing both halves of the event.