I think it was Loyce who said in a post somewhere that the best time is in the morning hours and especially during weekends when there are less business and institutional transactions.
You can look at
https://jochen-hoenicke.de/queue/#0,2w over the last 2 weeks (or longer if you prefer) to see pretty clear trends as to when there are big spikes in activity, and when the mempool largely clears out.
Generally speaking, surges in fees occur around 12.00 - 18.00 UTC. After this, they tend to plateau and then decrease, and usually are at a minimum around 01.00 - 07.00 UTC. Obviously the time for you depends on where you live, but if you live in the Americas then evening is going to be cheapest for you, while if you live in Asia then early morning will be better.