One thing I would add - Writing a gratitude journal once a week has been shown to be more effective than doing it daily (Sonja Lyubomirsky 2007)

I found two publications by Lyubomirsky in 2007. Only one contained something remotely similar to your claim:

Tkach (2005) demonstrated that participants who were randomly assigned to vary the types of kind acts they would perform on a weekly basis showed higher levels of happiness and well- being 10 weeks later relative to those who did not vary their kind acts, and relative to comparison controls.

However, this is about "kind acts", not about a "happyness journal".

Can you clarify the daily-weekly claim?