The US-30 Year and the US-10 Year Bonds both decline from the beginning of the year into a bottom around April / May every year, and from then on rising into October / November. Then a new decline begins. This is more or less the opposite of the stock market.
Yield of course moves opposite of bonds. On this 10 year bond yield chart the seasonal bias is relatively clear.
Euro Bund 10 Year is declining from the beginning of the year into a bottoms around April / May and from then on rising into January the following year.
GB Gilt is rising from the beginning of the year into April making a top and then decline into a bottoming around June / July, and from then on rising into April the following year.
Japanese Bond 10 Year is rising from the beginning of the year into May making a top and then declining into July / August making a bottom, and then a new rise into May the following year.