Best times of the day or week to publish auctions
If you are creating auctions, rather than Buy-it-nows, then generally you need to end your auctions when people are at home to bid on them last minute. For example, 7pm - 10pm in the evening would generally be good. Some people set alarms on their phones and bid last minute if it's something they really want and with popular items, then a lot of the bids can be in the last couple of minutes.
Telling eBay users you are on holiday/vacation
If you have a business account with eBay, then this is an eBay online control panel option. This isn't available if you are a private seller. If you have a lot of Buy-it-now auctions as a private seller, then you can update your descriptions to mention you are away. This works well if you only use the main description to tell people generally about your auctions and don't use it for information on the specific items. You can use the Bulk changes button on the 'eBay auctions' tab to update your general description. You can also use 'Item specifics' on the Bulk changes form to create a custom label called say 'On holiday' and fill in with something like 'On holiday until 14 December'. Item specifics are little tags to give extra information to users. The great thing about this is that they appear on the eBay mobile app when eBay users are using their smart phones.
What you can also do is to change the despatch time on your auctions. You might normally the the despatch time set to '1 working day' or '2 working days'. Just update this to say '10 working days' which means the that expected delivery time a potential buyer will see will be well ahead and they might end up reading your description to find out why. On mobile devices the description only appears if you specifically click on it.