The simple answer is "It's complicated". Not a very useful answer though^^
The more elaborate answer is that there are quite a lot of factors that play into this decision. For example, many years in the past, we had game worlds where the endgame happened on Easter, Christmas or other holidays that are important to many of our players. We learned from that and try to avoid server starts where the endgame happens on holidays.
Another "rule" is that we need a certain timeframe between the end of a game round and it's restart. First of all because the duration of the endgame can vary and secondly because we need some time to process the data from the finished game round. But everything beyond a week can become a problem, because that's when players start complaining, because their home game world is offline for too long.
Then there are workflow related reasons/rules. For example it's a problem if we have too many server starts on one day. Every server gets tested before it goes live and that simply requires time. Not that much time, but it piles up if there are too many server starts at the same days. When everything is alright this is usually fine, but in the rare cases where we DO find problems, multiple game worlds at once can become a bigger problem.
Often, multiple server starts at the same day are unavoidable, but we try to limit that wherever possible.
Additionally, we try to spread out server starts more or less evenly, because especially for new players it's best to have "young" game worlds available at all time.
Another rule we have is that we try to keep "work week" game worlds in the work week (i.e. starting somewhere between Monday and Friday) and weekend game worlds on the weekends.
And in a perfect world we would of course also prefer game worlds to always start at the same day of the week (although we also occasionally have players who are asking for the opposite, because they like the game world but the start date is inconvenient for them).
In reality, many of these rules tend to be in conflict with each other and there is no perfect solution, so we need to compromise. And that is very likely what happened to your game world.
edit: For the sake of clarity I should add that the reasons/rules above are just examples and not necessarily complete. I'm not personally in charge of creating the server start plan, so I am sure I forgot a few things.