I often see people asking about when, exactly, Imbolc is celebrated. The truth is the answer is a bit complicated. People see Imbolc listed in books as either the first of second of February, and figure maybe it moves around like the solstices and equinoxes do, and that's not always the case. For many, Imbolc is a fixed-date festival and does not move (although it does for some, more on that later).
I think part of the problem comes from the fact that the ancient Celts considered sunset to be the start of the new day, as several ancient cultures did. So, matching that to how we view our days, a holiday might start on the sunset of the 1st, and last until the sunset of the 2nd. (Or the 31st to the 1st.)
Another source of confusion comes from Imbolc versus Candlemas, and how the name is sometimes seen as interchangeable within neo-paganism. Imbolc is an old Celtic festival, and has strong ties to the goddess Brigid. Brigid became a saint in the Catholic church, and her feast day is still the 1st of February. On the other hand, Candlemas is a Christian holiday, which has ties to quite a few old pagan customs, and is still celebrated on the 2nd of February. Groundhog Day is another date that can be considered in all this, as weather divination would be popular around the time of Imbolc, and of course that is what Groundhog's Day is all about - which, again, falls on the 2nd.
To make matters more complicated, remember how I said the festival moves for some? Some choose to celebrate on the exact date between the winter solstice and spring equinox. Since the solstice and equinox shift from year to year, this means the date of Imbolc also moves when using this method. For those curious, this year that date is he 3rd of February.
Let's make things more complicated... some instead choose to celebrate on the full moon nearest the 1st, or when the first signs of spring actually show up, or traditionally with the blooming of blackthorn, or the beginning of the lambing season, which of course can all vary from year to year.
All this is a long way of saying that Imbolc is celebrated... well, whenever you or your tradition says it should be celebrated. Do you want to celebrate from the 31st of Jan to the 1st of Feb? From the First to the second? Just the first? Just the second? Or how about on the third, or the full moon, or...? It really comes down to the individual practitioner, and what makes sense for their tradition.