Depends on what you're dosing. If you're dosing with anything other than EI, probably anytime before lights come on. And, since Excel has a limited lifespan (24 hours is the best guess), if you use it as a carbon source, best to dose not too long before the photoperiod (rather than, for example, the night before).
However, if you're dosing EI, it really doesn't matter. The whole idea of EI is to make sure plants always have more than enough nutrients, so even missing a day or two won't make a difference. I dose dry ferts whenever I happen to remember during the day.