I think Sand Empire is one of the best times to use saved up EHT, since all of the event heroes for this one are strong and interesting, including the 3*.
Ranvir and Seshat are also strong HOTM to have a shot at while summoning during Sand Empire.
You could also consider saving them for December for Santa’s Challenge, since those heroes are arguably stronger (albeit unlikely to get).
There’s been some discussion of this over here too: What is the best Seasonal Summons?