What did the Buddha say about gratitude?
The Pali is 'kataññutā' &amp; also 'kataññutā kataveditā'. 'Kataññutā' means 'what others have done for us'. 'Kataveditā' means 'what we must do in return'.

Kataññu (adj.) [cp. Sk. kṛtajña] lit. knowing, i. e. acknowledging what has been done (to one), i. e. grateful often in combn with
katavedin grateful and mindful of benefits
Kataññutā (f.) [abstr. fr. last] gratefulness (defined at KhA 144 as katassa jānanatā)
Kataveditā (f.) [abstr. fr. last] gratefulness:
Pali dictionary

SN 20.12 states:

Tasmātiha bhikkhave, evaṃ sikkhitabbaṃ: ''kataññuno bhavissāma katavedino. Amhesu appakampi kataṃ mā nassissatī''ti. Evaṃ hi vo
bhikkhaveva, sikkhitabbanti.
Monks. You should train yourselves thus: &quot;We will be grateful &amp; thankful and we will not overlook even the least favour done to us&quot;.
This is how you should train yourselves.

AN 2.31-32 state:

Now what is the level of a person of no integrity? A person of no integrity is ungrateful &amp; unthankful.
I tell you, monks, there are two people who are not easy to repay. Which two? Your mother &amp; father. Even if you were to carry your mother
on one shoulder &amp; your father on the other shoulder for 100 years, and
were to look after them by anointing, massaging, bathing, &amp; rubbing
their limbs, and they were to defecate &amp; urinate right there [on your
shoulders], you would not in that way pay or repay your parents.
Mother &amp; father do much for their children. They care for them, they nourish them, they introduce them to this world. But anyone who rouses
his unbelieving mother &amp; father, settles &amp; establishes them in
conviction; rouses his unvirtuous mother &amp; father, settles &amp;
establishes them in virtue; rouses his stingy mother &amp; father, settles
&amp; establishes them in generosity; rouses his foolish mother &amp; father,
settles &amp; establishes them in discernment: To this extent one pays &amp;
repays one's mother &amp; father.

As for the word 'mangala', it means 'luck' or 'blessing' (rather than 'observances')

Mangala (adj.) [cp. Vedic mangala. Expld by Dhtp 24 with root mang, i. e. lucky; see also mañju] auspicious, prosperous, lucky, festive
Nd1 87, 88; KhA 118 sq.; SnA 273, 595; Sdhp 551. -- nt. mangalaŋ good
omen, auspices, festivity