Successes Scary? No.
Do they trigger depression? No.
Can holidays trigger? YES.

Holidays are the one of biggest stressors, next to romantic relationships to trigger depression. There is so much expectation, activity, busyness, attachment to ourselves as children, that, yes...they are very much a trigger.
So are anniversaries of wedding, death, divorce...as well as losses in general...loss of job, death in family, etc.

All of this depends on what the issues are of the person suffering from depression...whether it be an abusive childhood, abandonment, general mistreatment that was never dealt with...etc.

Each persons depression is as unique to them as our fingerprints. Each persons healing strategy needs to be as unique as they are...tailored to fit 'them'...their needs.