You have indicated your lawn is pretty long.

What is the prime reason your lawn needs cutting now?

Was the first portion of your comment accidentally deleted?

I ask because your very first sentence began with “So ...”

Heat doesn't have anything to do with whether mowing is needed or advised or not. Just mow appropriately high (as you should anyway) and keep the lawn watered as necessary.

Nope it wasn’t deleted. The prime reason is that it’s longer than I’d like it. I don’t want to mow it on the verge of a heat wave if I could potentially lessen the stress by not cutting it. It’s already growing slow and it might not be much taller after, hopefully.

I keep it high It’s mostly tttf but still looking a little beat up from the heat already in a portion where it gets afternoon sun

It depends and isn't a simple yes or no answer. If you irrigate or rainfall will be normal, then yes, go right ahead and mow.

If you don't irrigate and rainfall is on the low side, you can mow if you want to, but realize that it's likely to do a bit of damage and send the TTTF into its version of short-term dormancy a bit sooner. Leaving it a bit long won't hurt, just send it back down in stages later on when rainfall resumes. Mow it as long as you can to leave the dying or dead blade there to shadow the crown and soil as much as possible.

With fescue (and rye), occasional irrigation is recommended; it really doesn't have much in terms of a dormancy mechanism, it just slows down and drops its blade to preserve the crown. Make sure it gets about a quarter inch of water every two weeks one way or another throughout summer.

Bluegrasses differ.

If you mow drink lots of water. I tried with out it and spent the night in the hospital with heat exhaustion

@krnuttle: So you're the one! Funny because every time I work outside in heat like yesterday I think about some guy on the lawn forum who told a story about getting heat exhaustion after working outside all day. Now I have a name to go with the sentiment!

I can not believe that I am the only one who did a dumb thing like that.

I don't know if it was me, but if it was I am glad my story has help someone avoid what I went through.