These have been happening on a cycle for around 6 months now, but I wouldn't say there's a precise schedule, or at least, not one that is publicly available.

You could probably expect a 2x sacred event again in the next few weeks. We had a Void 2x event recently, so that one will happen again last, most likely.

If you're wondering whether to use your sacred shards or wait for an event, that really depends on what you want or need. You could get a lot of points in the current Champion Chase Tournament, but if you want the highest chance possible for legendary results, wait for double rates.