There is actually an existing tool available online made to answer your question. The tool can be accessed here: https://joticajulian.github.io/steem-activity/index.html
It seems the biggest upvotes happens on Saturdays at 10 PM (22H) according to the tool above.
And here is a more detailed summary per day. It's all just numbers and graphs but you could get an estimate there on what time curators are mostly awake and curating. Good Luck! :)
Do you mean time of the day or time in one's blogging time on steemit. If you mean time of the day then I guess it depends on when your biggest upvoters are online. Posting the time when they are awake and online will likely fetch you high upvotes. Some love to post in the morning some noon and some night, because they have a target audience.
If you meant on your steemit journey then that would be the day you make a post that catches the interest of many or even just one whale and he upvotes and reesteem your post to his or her followers. You are in for a shocker upvote.
When people pay for bid bots.
From what I have been told it is on the weekends around 10:00. Mainly saturday. I assume it has to do with people being kinda drunk on the weekend and looser with their upvote button.