Xanathar's Guide to Everything has a 1st level cleric and paladin spell called Ceremony, which has a number of various options which correspond to things which have traditionally been sacraments of the church in the real world — weddings, funerals, coming of age ceremonies, atonement, and etc. One of these options is:
- Bless Water. You touch one vial of water and cause it to become holy water.
This spell has a casting time of one hour and every option requires 25 gp worth of powdered silver, which the spell consumes.
This corresponds perfectly with the "special ritual" described in the basic rules, under holy water, which:
takes 1 hour to perform, uses 25 gp worth of powdered silver, and requires the caster to expend a 1st-level spell slot.
So, while @sadaqah's answer is correct pre-Xanathar's, it seems that they've decided to fill in this missing space with a specific spell.