The city is well-served by a variety of hotels, though budget hotels are few and far between. The one exception is the growing chain of Omenahotelli (look for the big apple): http://www.omena.com/frontpage.
Retkeilymajat (youth hostels) are used by travelers of all ages, and they are almost always clean, modern and quite comfortable. Most of the major hotels are near the railway station, with a few spread farther afield. Most of the hotels outside central Helsinki are close to one of the several major exhibition and conference centers, where local amenities such as shops, bars and clubs are sparse. The center-city hotels tend to be better.
In such a busy city, with conferences occurring throughout the year, there is no one time when rooms are particularly scarce, though the summer tends to be a little more crowded than winter. However, at every season it is wise to book well in advance. Note that many hotels close for Christmas, some remain closed through the New Year, and a few close for 1 May and Midsummer, a major holiday celebrating the summer solstice.
