According to Diodorus Siculus, the continental Gauls sacrificed humans as a form of religious divination (like reading tea-leaves or interpreting weather) :

These men predict the future by observing the flight and calls of birds and by the sacrifice of holy animals: all orders of society are in their power... and in very important matters they prepare a human victim, plunging a dagger into his chest; by observing the way his limbs convulse as he falls and the gushing of his blood, they are able to read the future

Julius Caesar's Commentaries offers another explanation for Gallic human sacrifice; the Gauls were an incredibly superstitious people and routinely sacrificed people to cure their ills or please their cruel gods, often in a brutal fashion:

The entire nation of the Gauls is very given to religious scruples, and for this reason, those who are affected by grave diseases, and who take part in battle and in peril, either sacrifice men as victims or vow to sacrifice them, and they use druids as the performers of these sacrifices, because they believe that, unless for the life of a man, the life of a man is returned, the power of the immortal gods is not able to be appeased, and they have instituted sacrifices of this kind for the public. Others have images great in size, the limbs of which, interwoven with twigs, they fill with living humans; the men, with these having been set aflame, perish. The punishments for those apprehended in conspiracy or in thievery or in other crime are thought to be most pleasing to the immortal gods; but, when abundance of this kind fails, they even defer to the punishment of the innocent.

As far as I'm aware, there is no evidence outside of Caesar's Commentaries that corroborate his claims about filling giant wicker men with people and lighting it on fire. This may well have been hearsay or rumour that he picked up and included in his account. His description of human sacrifice seems pretty propagandistic, too; while we do have physical evidence of human sacrifice by continental Celtic speakers, it is unlikely that they just cut some random person's belly open whenever they had a bad day. Caesar's description paints the Gauls (and their druidic caste) as simplistic and incredibly superstitious, which seems to have an ulterior motive.

In my own area of study, human sacrifice appears to have had a political as well as religious dimension. Because Ireland had no conquering Roman generals or Greek ethnographers, we have no written accounts of human sacrifice (or any of the island's religious practices and beliefs) there. However, a recent archaeological theory has proposed that many bog bodies found in Ireland were sacrificial victims of a ritual pre-Christian coronation ceremony, and not victims of murder or inter-communal violence as had had previously been theorized.

Researchers at Ireland's National Museum of Archaeology have examined several bog bodies found in Ireland and concluded that most of the individuals found were victims of a ceremonial inauguration for Ireland's Iron-Age sacred tribal kings of the tuatha, an archaic social structure which were surpassed in importance and subjugated by inter-regional dynastic kingdoms. Legal texts show us that these kings had a mostly symbolic importance and had extremely limited political or judicial powers. Evidence from the historical period of Irish history show that pagan, ceremonial inaugurations survived for centuries, with accounts of newly inaugurated kings marrying the personification of their territory (probably originally the region's pagan deity) and having animals sacrificed before consuming a broth made from their bodies. It is possible that animal sacrifice replaced human sacrifice at an earlier date, but it's also possible that the two are unrelated ceremonies as animal sacrifice usually held some distinct ritual importance.

The actual ritual of human sacrifice in early Ireland took place on the boundaries of these tiny tribal kingdoms, and have been theorized to have been a statement and definition of the new king's authority. The human victim was accompanied by prestige goods such as royal regalia, utensils, equestrian equipment, bits of weapons and shields, items for food production and even butter. Celtic speakers seem to have regarded bodies of water as sacred places in general, and many important La Tene material goods like the Battersea Shield have been recovered from bogs, streams or lakes. They likely believed that these were liminal places, where the corporal world and a spiritual other-world could overlap, which explains their religious and ceremonial importance. Since we have no textual evidence to corroborate these early sacrifices, we can only really guess that these were religious sacrifices that were a statement and confirmation of a new king's sacred and political authority.

So to answer your question more directly, it depends on the region. Classical accounts make it seem like the continental Celtic speakers practiced human sacrifice quite regularly, but Insular Celtic speakers in Ireland appear to have practiced human sacrifice fairly infrequently (as far as we know, there are probably way more undiscovered bog bodies) as part of a sacred coronation ceremony.