I believe to have been that kind of customary payment or oblations made on Sundays to the rector, or his vicegerent, of the church where a person heard divine service and received the sacraments:
"Hostiensis dicit quod in præcipuis festivitatibus tenetur quis offerre, et cogi potest; maximè cum sit quasi generalis consuetudo ubique terrarum ... et intelligit festivitates præcipuas, dies Dominicos, et alios dies festivos."—Lyndwood, Prov., p. 21., not. e., ed. Oxon. 1679.
Though Lyndwood himself, as I understand him, seems to doubt the cardinal's statement, that the payment could be enforced, unless sanctioned by local custom.
Ducange, in v. "Denarius," 8vo. ed., Adel. 1774, says, the "Denarius de Palmâ" and "Denarius Dominicalis" were the same:
"Habebit (vicarius) cum eis victum suum competentem, et ad vestes sibi emendas XL. solidos Andegavenses, et Denarium singulis diebus Dominicis ecclesiasticâ consuetudine offerendum."
On this extract from a charter he observes:
"Erat itaque Denarius de Palmâ, ille qui singulis Diebus Dominicis et [lege à] fidelibus offerrebatur. Cur autem dictus 'de Palmâ' non constat, nisi forte sic dictus fuerit quod in manum seu palmam traderetur." Denarius Dominicalis, idem.—Arest. MS. a. 1407.
It would seem also from his definition to be the same as the payment called "Denaria Sacramentorum," that is:
"iidem denarii qui singulis offerrebantur Dominicis, ideoque Sacramentorum dicti, quod tempore Sacrosancti Missæ Sacrificii, pro excellentiâ interdum nudè appellati Sacramentum, a fidelibus offerrentur."—Annal. Bened., t. iv. p. 466., n. 80. ad annum 1045.
These extracts sufficiently explain, perhaps, the payment known by the different names of "Dominicals," "Palm-penny," and "Sacrament-pence;" and still indicated, probably, by the weekly offertory of our communion service.