If you’d like to take a conservative submissions approach, divide your agents into buckets: A list, B list, and everyone-else list. Try submitting in rounds of 4-8 at a time (depending on the size of your list), including 1-2 of each agent type. If your A list people immediately and favorably respond, then I’d send out another round right away, a mix of As and Bs, to see if you can gin up competing interest. If responses trickle in with no particular pattern or order, send another round within 2-4 weeks or so. At least every month, send another round until your list is exhausted.
If you immediately see a pattern in the response that indicates something’s amiss, you can adjust your approach for the next round of queries. The reason I recommend this conservative approach is it tends to be easier to manage psychologically. But there’s nothing wrong with sending out your materials to everyone on your list at once. It just means that you don’t get that “next chance” or opportunity to adjust your pitch later. (Once a rejection, always a rejection—or that should be your assumption.)
There is no best time to submit to an agent or publisher, aside from advice offered in submission guidelines. If an agent or publisher is closed to submissions, it will likely be a waste of everyone’s time to submit. If you send your materials anyway, they are likely be ignored, deleted, or discarded without response.
There is also no “bad” time to submit, despite common wisdom you may have heard. E.g., some say you shouldn’t submit over the holidays or in August (when publishing tends to go on vacation). But every agent and publisher is different. What’s a good time for one is a bad time for another. There is no universally great or bad timing that works in every situation. However, it doesn’t hurt to check the social media feed of the agent or editor to see if they’ve offered any advice or hints about submissions.
In the end, the best time to submit is when your materials are ready and you feel ready—sufficiently focused and engaged—to manage the process.