The science
A randomized trial comparing different rehydration drinks found that electrolyte content, especially sodium, was the single biggest factor in how much fluid your body actually holds onto. Drinks with sodium in them were retained significantly longer than the same amount of plain water (PubMed, Maughan et al.).
So what
There’s no secret golden hour here. The honest answer is: whenever you’re already running a deficit. That’s first thing in the morning, after seven or eight hours of not drinking anything while you sweat through the sheets and breathe out moisture all night. It’s also after a hard sweat, a day in the heat, or the morning after wine did its diuretic thing to you. Matching your intake to when you’re actually losing fluid matters more than hitting some specific hour on the clock.
Bottom line
If you only remember one time, make it the morning. You’re already behind before you’ve had a sip, so it’s the easiest habit to keep. Have it before your coffee, not as an afterthought once you’re three cups in.
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