little bit individual and geographical... depends on the country, age of horse, training (maybe the horse does not work cos tendon treatment) and you can wait till August cos healed yet.
and horse racing barns do not clip most of their horses in hot countries. A healty and full training horse loses tons of hair while grooming. So if you plan to sell clipping tools dont do this business in New MExico
UK weather is so unpredictable it often three seasons in week, most horses/ ponies lose all their winter coat by the 1st June, then start losing their summer coat straight away.
If you have a horse on a racing yard it may never get a full coat because it’s in and rugged if it’s a flat horse but hurdlers and steeple chasers do get turned out, so often they are the ones that are more noticeable when they clip
It could be hay making weather in October. My daughter was born Burghley week, I missed it that year. Before she was born it was never that hot a week but now unless it rains you dress for good weather and sunscreen is a must.