Gracias sits at around 800 metres elevation and has a pleasant highland climate. The dry season (November through April) is generally the best time to visit — trails are passable and the days are sunny. The wet season (May through October) brings daily afternoon rain that can make the Celaque trail slippery and the summit views rare.
For the Celaque hike specifically, February and March offer the best conditions: dry trails and higher chance of clear summit views. The town itself is worth visiting year-round; a bit of highland rain doesn't diminish the colonial streets.