Summer bonus

My company pays it out on the 15th (so does my husband's), but I think it is by the end of May.

End of April.

Some pay in April, others in May and the bulk at the end of June. It's the result of bargaining agreements and/or company culture

Legally it needs to be paid when the majority of the annual holidays (the longest period of continuous holiday) is taken. In practice (to avoid having to pay on different time to each employee) most of the employers pay the bonus to everyone in June, just before the summer holiday season (=school summer holidays) starts.

"Legally" it does not need to be paid at all. Nor does the 13th month in December. However if you are entitled to anything under a collective bargaining agreement, it will be defined in your contract of employment.

Unlike the 13th month the summer bonus is legally binding. It is actually not a bonus at all, but a holiday allowance defined in the labor law (double pecule de vacance).
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As holidays it is accrued during the year preceding the year when the holidays are taken. Assuming you worked full time the whole year N-1, on year N you are entitled to the allowance worth 92% of one months gross salary. Even thought the accrual is during N-1 year, the reference salary for the calculation of the holiday allowance is the salary of the month when the holiday is taken.
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For part timers or those who worked only part of the year the amount is calculated pro rata. Interims receive the double pecule as the simple pecule (the accrued paid holiday time) in their normal weekly salary as advance payment. In practice they receive about 15% extra pay, but then do not get paid whilst on holiday). Employees in the public sector have their own system.
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The system for blue collar employees (ouvriers) is slightly different. The allowance is paid by the 'Caisse de vacances', not employers and the amount is about 15% of the gross salary of the accrual period.
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The holiday allowance is subject to social security payments + taxes
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To go even deeper on this, the new legislation on the so called European holidays is financed with the holiday allowance. The new comers on the Belgian labor markets now have a right to take paid holidays after 3 months of employment. Since they do not have accrual via employment in Belgium on the year N-1, their holiday days are paid as advance of the holiday allowance. Correspondingly the amounts paid in advance are deducted from the holiday allowance on the coming years.