Sunday because you need less upvotes than other days to reach the top.
12:01am PST to give your product the most amount of time possible to reach the top of the charts for the day.
Use emojis in the descriptions and taglines of their products. The emojis will jump out at people compared to text alone.
Ask people for upvotes. The people at Product Hunt do not like this. They will make your product less visible if they catch you asking for upvotes. Instead guide people towards your post by letting people know you've launched on Product Hunt.