Basic rule is:-
After maintenance - All 4 day dragons spawn within 24 hours after the server goes live. Why this happens, is because in the original release of the game, ALL dragons would be up as soon as maintenance ended so Runewake botched it quickly by adding a random 24 hour timer when the server goes up. Had one afternoon were spawns were slept on and I needed compendium completion at the time, so I ended up with about 8 wild ancient captures in 30 minutes. (This doesn't include moon focused dragons, or solar eclipse, unless it happens to be the right day/phase when the servers go up.)
After that initial spawn, it goes to a random 3-5 day timer which makes them a bit harder to predict and find, so after maintenance is the best day to find them if you don't want to be hung up waiting several days for them.