There’s an old proverb that says something along the lines of “The best time to plant a tree was 20 years ago, the second best time is now!” and the same can be said for nestboxes. If you can get them up in March there’s still a good chance they’ll be used that year, but the further in advance of spring the better.
If you put it up in the middle of the nesting season (summer months) it’s unlikely to be used until the following year as birds will have already established territories and found nesting sites. By putting it up well in advance of the following breeding season you’re giving your local birds a chance to find it, get used to it and check it out ahead of the following year. Also, small birds such as Wrens will often roost (i.e. sleep) in nestboxes during cold winter nights, as somewhere warm and sheltered – so it’s not all about nesting when it comes to nestboxes!