The best time to prune most roses is in late winter to early spring, just as new growth starts. This timing can range from as early as January in milder areas to around March or April in colder regions. For example, in the UK, spring growth may start as early as January in the south but not until April further north. In the US, a common rule of thumb is to prune when forsythias bloom, which often signals the swelling of rose buds in early spring. Different types of roses may have slightly different pruning times:

• Most roses: Late winter to early spring before new growth starts.
• Rambling roses: Usually pruned in summer after flowering or sometimes in autumn/winter after leaves drop.
• Shrub roses and some old garden roses: Light pruning in late winter (February to March) and deadheading in summer.
• Repeat-flowering roses: Prune back previous season’s growth by 30-50% in late winter.

It's important to avoid pruning too early if there's a risk of frost, as fresh cuts and new shoots are frost-sensitive. Also, prune to remove dead, diseased, or weak stems and to open up the plant for better air circulation and healthy growth. In summary, prune your roses late winter or early spring, timed with new growth or blooming of forsythia, unless you have specific varieties like ramblers that need summer pruning after flowering.