Based on the Japanese translation, it's supposed to be "I am serious especially times of rest".
Oh. I see it now. The Japanese got translated into broken English as an intermediary ("I serious for rests") before ending up as a word-by-word swap into Ratin.
Gravi(s) (literally "heavy") is especially bad because it means "serious" as in "grave", "solemn" or "important". A "weighty situation" or someone/some thing of "weighty importance", in other words. It's more of the impersonal kind of "serious", putting it closer to 深刻な or 重大な (or 重 in word compounds).
The "earnest" kind of "serious" is probably best translated as "studiosus" (diligent, earnest, assiduous, studious) or "ardens" (passionate, ardent).
"Sincerus" (sincere) would probably be a more precise translation of 本気, but it doesn't fit as well here.
In any case, "Inter ferias studiosissimus sum" could work. Not as clear in meaning though (compared to the original Japanese), and my Latin is a bit rusty. There is also "Inter ferias studiosissimus es", which matches the imperative in the original Japanese, but this sounds weirder.
The pithy way of saying it (which also gets most of the point across) would be "Carpe ferias!". Seize ("pluck") the holiday(s), like "seize the day", as in "make best use of your holidays/times of rest).
Knight in Awai-mura's town hall reception deskAlas, I fear not. However, I bear a certificate of my nobility from my ancestors and my family's coat of arms in stead. Our motto has changed recently: "Determination in times of rest" I have been told I must tend to the proceedings for immigration. I would like for this to be arranged....Do you have a change of address certificate?Motto
Often on the bottom of a coat of arms, a place where one could write anything they want.
Even writing your favorite sushi topping is probably okay
